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February 21

Mercury thermometers

Mercury thermometers become more inaccurate over time. Do they record (overly high) tempatures, or overly low tempatures when they are too old?149.152.23.48 (talk) 00:36, 21 February 2013 (UTC)

I added a title. StuRat (talk) 00:43, 21 February 2013 (UTC)

Note that, if they did drift, all they would need is to be recalibrated (have new numbers painted on). StuRat (talk) 00:45, 21 February 2013 (UTC)

Who says they loose accuracy over time? Accuracy is determined by three things: Random error in taking readings - in the conventional mecury thermometer this is merely a factor of how carefully one looks at the scale and compares it to the mercury height; Systematic error, which is determined by the accuracy of calibration (the position of the glass tube with respect to the scale, accuracy of capilliary diameter), and drift.

In metrology generally, drift is made up of thermal drift, inapplicable to thermometers as it is taken up in calibration, drift over time, and noise. There are no significant time and noise drift mechanisms for normal sealed mercury-in-glass thermometers.

Possibly the OP is referring to sensor thermometers that were once used in photographic processing labs, chemistry labs, and industrial control systems. In these, the mercury is electrically earthed via a wire sealed into the glass at the bulb end, and there is a probe wire fed in via a hole in the glass at the top end. The probe wire is adjusted in position so that at the desired temperature, it just touches the mercury column. When it does, it closes an electrical circuit, which controls the tank heater or whatever is required. These thermometers, when new, are extremely accurate compared to alternatives at the time. However, as the probe wire must move freely in the hole, there is not a perfect seal, and over tens of years some mercury vapour can escape to atmosphere, leaving less in the tube. So, over time, the thermometer reads lower and lower compared to the calibrated temperature. These wire probe type mercuray thermometers have been obsolete for 40 years or more, as solid state electronic thermometers (quartz or platinum sensing) were developed with more than adequate accuracy for the purposes that a mercury thermometer would otherwise be used. But you still sometimes see them. I saw one still in use in a photographic lab only a few years ago.

Ratbone 60.230.238.153 (talk) 03:14, 21 February 2013 (UTC)

In general, mercury thermometers do not drift. However, they may become damaged. Sometimes an old thermometer will have discoloration of the mercury or even bits of oxidized mercury along the capillary. This is caused by improper filling, where moisture or air was sealed inside the instrument. On high temperature thermometers, sometimes a portion of the column evaporates and then condenses in the upper portion of the thermometer. If a thermometer is treated roughly or thermally shocked, it can develop a nearly invisible stress crack, allowing mercury to escape. because of these effects, mercury thermometers need to send out for calibration, but the calibration is not because of any natural drift but because a close visual inspection under a magnifier followed by checking the accuracy is the most reliable way of identifying the above problems. --Guy Macon (talk) 08:27, 21 February 2013 (UTC)

We have several Mercury-in-glass thermometers provided by Environment Canada. Every one of these comes with a correction chart made up by EC. The oldest card and thermometer we have is dated 1978 and is still in use. There are two possibilities that could lead to inaccurate readings. First is if the column of mercury was to separate into two columns. Second, mercury freezes at −38.8 °C (−37.8 °F) and leaving them out at temperatures colder than that may cause problems. I wonder if the OP is thinking of mercury barometers rather than thermometers? It's been a long time since we used one but I seem to recall that the inspectors would check them out every time they were on a site visit. Also the mercury could be drained out for shipping which indicates that, unlike the thermometers, they were not sealed units. CambridgeBayWeather (talk) 01:02, 22 February 2013 (UTC)

Signs of life on Titan and Europa?

Do any of life currently exist on Titan. I thought Titan is way too cold to even support life. Its average surface temperature is -185 C/-300 F. I know Titan currently have water and rains. Some scientist thinks life could exist on Europa's ocean possibly sharks or maritime life. i wonder how is that plausible since Europa's surface is too cold -160 C/-260 F. Does this matter if planet fits under habitable zone to be habitable ? Or most important factor for habitable planets is water and atmosphere?--69.226.39.147 (talk) 01:22, 21 February 2013 (UTC)

If NASA had discovered unambiguous signs of life, I'm sure we would know about it. It is possible for life to not exist on the surface, and to only exist deep underground (or underwater). On Earth, we both have bacteria which live deep underground and live off minerals there and whole ecosystems around hydrothermal vents on the ocean floor, which get their energy from places other than sunlight, such as radioactive decay. Tidal forces could provide energy on the moons of the Jovian planets. If the energy sources are quite limited, you might find the plants and animals there have a much slower metabolic rate, and therefore grow and move more slowly. StuRat (talk) 01:38, 21 February 2013 (UTC)

We have an article on life on Titan. The corresponding article for Europa is a subsection only, mostly because there's virtually no hard evidence to go on (we have attempted no landings there). Due to the good chance of Europa having a subsurface liquid water ocean, my understanding is that it's considered to be a stronger case for extraterrestrial life than Titan; we just don't have as much evidence to examine. I note particularly that I find Stu's initial comment a little vague, mostly because of the various ways it can be interpreted. The Cassini-Hyugens mission found data from Titan that could be the result of biological processes, but could also be from other natural processes. Is that a sign of life? Maybe, depending on your definition. Even this tenuous bit should cast doubt on a blanket claim of "if there were, I'm sure we would know". — Lomn 01:50, 21 February 2013 (UTC)

Ok, let me add the word "unambiguous". StuRat (talk) 01:54, 21 February 2013 (UTC)

Even then, there are vagaries regarding just what "signs" of life are, and whether they're actually related to life, or whether "we" is "we the human race" vs "we the ref desk". Mostly I'd suggest sticking to a more straightforward referenceable statement like "Scientists do not currently know of extraterrestrial life on Titan or Europa, though it has not been ruled out" -- particularly for a leading sentence -- but that may just be the pedant in me. — Lomn 02:03, 21 February 2013 (UTC)

Oh come on, we don't need to be so pedantic, especially if it has a chance of confusing the OP. An unambiguous sign of life would be if a probe sampled the ocean on Europa and saw non-terrestrial microbes with its microscope. A even more unambiguous sign would be if a giant squid attacked the probe with its tentacles, and the probe took a picture of it. In any case, for any reasonable definition of "unambiguous", nobody has detected unambiguous signs of past or present life anywhere except Earth. --140.180.243.51 (talk) 03:37, 21 February 2013 (UTC)

For an interesting fictional treatment of life on Europa, see 2010: Odyssey Two. --Jayron32 02:01, 21 February 2013 (UTC)

"I know Titan currently have water and rains."

Titan has lakes and rain, but they're composed of simple hydrocarbons like liquid methane and ethane, not water.

"i wonder how is that plausible since Europa's surface is too cold -160 C/-260 F"

Both Titan (moon) and Europa have global subsurface oceans. Titan is 50% ice, which corresponds to many orders of magnitude more water than exists on Earth. Europa has a more rocky composition, but it still has a layer of liquid water 100 km thick, which corresponds to 2 times the mass of water on Earth. As for why these subsurface oceans can exist, it's mostly due to tidal heating. The moons orbit close enough to their planets that their near side feels significantly more gravitational force than their far side, causing them to be stretched. This stretching releases heat through friction, which melts enough ice to maintain a liquid ocean. As you said, the surface is too cold to support liquid water, but the interior fares better because the outer ice layer serves as insulation to prevent tidal heat from escaping. --140.180.243.51 (talk) 03:37, 21 February 2013 (UTC)

For another Wikipedia article on a plan to put life on Europa see Terraforming of Europa. Graeme Bartlett (talk) 10:22, 22 February 2013 (UTC)

Time running slowly

When the velocity of a body increases, time for that body starts running slowly. Also, when gravitational force increases, time, in this case also, starts running slowly. Here, in both cases time runs slowly. Why don't time run faster when velocity or gravitational force increases? Chi Yang --27.62.106.130 (talk) 01:58, 21 February 2013 (UTC)

See time dilation for an explanation, it isn't a simple as your original assumptions. --Jayron32 02:39, 21 February 2013 (UTC)

It is fundamental property of time. It is described accurately by spacetime which is a mathematical model that combines space and time in single entity. Gravity and velocity bends spacetime which is observed as time dilation. - manya (talk) 03:24, 21 February 2013 (UTC)

The time dilation for a moving object is relative - something not moving at that velocity sees the object's time slowing down, but the object itself (or anything moving the same speed in the same direction) does not see his time slowing down. Bubba73 ^{You talkin' to me?} 03:40, 21 February 2013 (UTC)

Calculating the electric potential between two point charges

When given the coordinates and the charges of two points, how do I calculate the voltage/electric potential present at some other coordinate? — Melab±1 ☎ 03:09, 21 February 2013 (UTC)

This is one of Maxwell's equations, specifically Gauss's law IIRC. --Jayron32 03:23, 21 February 2013 (UTC)

Gauss' law doesn't tell you the potential. See electric potential instead, specifically "electric potential due to a point charge". Potential is an additive scalar, so if you have the potential due to 1 charge, you can just add the potential due to the other. --140.180.243.51 (talk) 03:39, 21 February 2013 (UTC)

Palladium Hydride

So I am trying to figure out at what temperature Palladium Hydride releases Hydrogen? And at what temperature it adsorbs Hydrogen? How much Hydrogen can it release? Ender Wiggin (talk) 03:58, 21 February 2013 (UTC)

Did you read Palladium hydride? This suggests that you can get 7 hydrogen atoms for each 10 palladium atoms at normal pressure. About 6.6 grams of hydrogen per kilogram of palladium. At 300 degrees hydrogen will diffuse through. So I think that it will absorb and release at lower temperatures dependent on hydrogen pressure. It sounds like we need a graph. Graphs are available at https://notendur.hi.is/egillsk/stuff/annad/PdHyd_paper.pdf and http://www.platinummetalsreview.com/pdf/pmr-v4-i4-132-137.pdf These show that at 70° it can absorb hydrogen at 0.1 atmospheres pressure and then give it back at about 1 atmosphere at 145°. Graeme Bartlett (talk) 09:45, 22 February 2013 (UTC)

Pluto to have a watery surface

This is true in 7.5 billion years scientists eventually Pluto and Kuiper belt object will eventually reach habitable surface temperature [1] it shows that icy moons of outer planets can actually become a ball watery beach and it even said in 7.5 billion years as sun's giant Pluto and Charon will become a liquid beach. However most people will think when they hit habitable surface temperatures they will have technical problems is atmosphere driven off into space. (I was forced to learn when Titan and Europa hit habitable surface temperatures they will hit technical problems:Atmosphere driven off into space) Actually [2] artist's conception's image shows when Pluto warms up slightly, it can create a more substantial atmosphere, but the question will be how warm can the atmosphere survive on Pluto. Can Pluto's atmosphere survive when it gets warmer than the conditions Jupiter and Saturn's moon is now? Alot of artist conception shows when Europa gets warmer, I've watched it on Youtube video, Europa's ice can actually melt into ocean beach, but I am not quite sure on how long will the atmosphere survive there? --69.226.39.147 (talk) 05:17, 21 February 2013 (UTC)

Determining asteroid's orbits

I'm looking for a description of how Asteroid's orbits are measured/determined, and trying to get some idea of the accuracy that can be obtained. It seems pretty surprising to me that the approach distance to Earth can be worked out so far in advance, when surely there must be some close encounters with other bodies that can't be predicted, apart from the questional accuracy of the initial sightings. Thanks in advance. 124.191.176.222 (talk) 07:19, 21 February 2013 (UTC)

Orbit determination may help. Duoduoduo (talk) 20:10, 21 February 2013 (UTC)

Thanks, that was a help, but still no mention of how accurate the determinations are.124.191.176.164 (talk) 07:01, 22 February 2013 (UTC)

Close encounters with large bodies are very rare and predictable since we know where the large bodies are. Close encounter's with smaller bodies are still rare enough and have negligible effect in the short run. In short, you have a valid point but the time scale it takes for the unpredictability of the orbit to become a problem is much larger than what you imagined. Dauto (talk) 00:39, 25 February 2013 (UTC)

If Einstein hadn't invented the theory of relativity, would someone else have?

if einstein hadn't invented the theory of relativity, would someone else have? or might we still not know it today (even though we would know about quantum mechanics, for example). for example simply due to lack of interest or attention. (since we were focused on other aspects of physics for example). 178.48.114.143 (talk) 07:48, 21 February 2013 (UTC)

You never know. I could be the one, if not Einstein. ;) ☯ Bonkers The Clown \(^_^)/ Nonsensical Babble ☯ 07:55, 21 February 2013 (UTC)

No, this could never be. Long before peak oil and peak copper, peak intelligence already happened around 1900. Intelligence is steadily declining since. 95.112.148.69 (talk) 12:26, 21 February 2013 (UTC)

To be pedantic Einstien did not invent the theory of relativity. He postulated it. -- Alan Liefting (talk - contribs) 08:16, 21 February 2013 (UTC)

To be pedantic, it's Einstein, not Einstien. -- Jack of Oz ^[Talk] 19:32, 21 February 2013 (UTC)

(edit conflict) As for the question, we could compare it to the classic case of two separate people putting forward the theory of evolution, namely Charles Darwin and Alfred Russell Wallace. Einstein did have an amazing insight but there is nothing to suggest that at some stage (maybe decades later) someone else would came up with the same theory. Scientific discoveries and technological developments that after Einstein postulated his theories would mean that it would be easier for other scientists to arrive at the same conclusions. -- Alan Liefting (talk - contribs) 08:24, 21 February 2013 (UTC)

• I believe that most physicists think that Special relativity would soon have been invented if Einstein hadn't -- other people were very close to it. The story may be different for General relativity -- that was a huge leap beyond anything that anybody else was doing, and even now there are only a small minority of physicists who really understand it. Looie496 (talk) 08:20, 21 February 2013 (UTC)

  All basic ingredient of special relativity had been known from works of Hendrik Lorentz and Henri Poincaré before Einstein. So, Einstein only made the last step. General relativity would have likely been discovered without Einstein as well—David Hilbert obtained Einstein equations almost in the same time and independently of Einstein. Ruslik_Zero 12:17, 21 February 2013 (UTC)

  Don't forget Hermann Minkowski who also provided much of the mathematical underpinning for both relativity theories. Minkowski basically worked out all of the geometries of spacetime prior to Einstein applying it to light (special) and gravity (general) relativity theories. Einstein did not work in a vacuum, and it is possible, even quite likely, that with all of the ingredients someone would have put it all together eventually. Unprovable, but also not out of the realm of possibility. As Isaac Newton once famously noted "If I have seen further it is by standing on the shoulders of giants." Such a sentiment applies just as well to Einstein as anyone in the sciences. --Jayron32 15:06, 21 February 2013 (UTC)

  I'm not aware that Minkowski did any work on curved spaces. Regarding Hilbert, that's true, but it raises an interesting sociological question: he was considered a pure mathematician, and it's unlikely that any physicist would have paid serious attention to his ideas (since none of them could understand the mathematics). Einstein, in contrast, was already the most famous living physicist when he presented General Relativity, due to his incredible 1906 papers. Looie496 (talk) 17:01, 21 February 2013 (UTC)

  See Minkowski space, Minkowski diagram, etc. --Jayron32 18:45, 21 February 2013 (UTC)

1905. -- BenRG (talk) 19:25, 21 February 2013 (UTC)

I tend to agree that Minkowski's discovery of spacetime was a more important breakthrough than Einstein's original paper, but Minkowski's famous talk was in 1908 and I'm pretty sure it was Einstein's paper that inspired him to work on the problem. Minkowski died in 1909 and didn't do any work on general relativity as far as I know. Einstein initially didn't like the idea of spacetime but later said that he couldn't have discovered general relativity without it. -- BenRG (talk) 19:25, 21 February 2013 (UTC)

Indeed. Minkowski was Einstein's professor in Switzerland, and the two remained life-long collaborators, frequently critiquing and adding to each other's work. Minkowski provides the intermediary link between the special and general relativity theories: he developed his spacetime geometry as a way to simplify the mathematics behind special relativity; Einstein in turn used his unique four-dimensional geometry as a launching-off point to come up with his geometric description of gravity which forms the core of General relativity. These discussions are not to discount the singular genius of Einstein. It may have taken a dozen different people another dozen years to come up with what he did in the course of a much shorter time. But to say that doesn't mean that, had he not existed, all of those principles would simply never have been discovered or elucidated at all. He did not work in a vacuum, and had he not existed, others would have filled that void. Science works by answering the as-yet-unanswered questions (and also, it should be said, by asking the as-yet-unasked questions) and had Einstein not answered them, it seems impossible that no one else would have in the intervening years. --Jayron32 20:26, 21 February 2013 (UTC)

So far, the responses have focused on "how close" others were to Einstein's general theory. It sounds like the person who was closest, Hilbert, was doing pure math. That leaves the question of whether - even if someone had equations close to Einstein's - anyone would have given the specific thing great attention. (For a moment, let us say that Hilbert moved on to other things or whatever, like leaving equations in a notebook). Is it "natural" to develop the general theory of relativity, from the things that everyone was already looking at, such as the speed of light? Was there any particular reason to look deeply into that direction? What I mean to say, is that it is easy for me to imagine that, as quantum mechanics developed as something that physicsists could experiment with, and as in a practical sense everyone uses QM every day in electronics, etc, it is easy for me to imagine that we have QM today but nobody has invented general relativity. That would only be possible if it were possible for physicists not to really focus on the space that General Relativity is in . Could that have happened? How obviously was it in need of physicists' attention? 178.48.114.143 (talk) 18:39, 21 February 2013 (UTC)

From a modern perspective general relativity is the only relativistic field theory satisfying some pretty simple constraints (spin 2, no derivatives higher than the second... that may be all), so I'm sure it would have been discovered at some point. I'm actually kind of surprised it took Einstein 11 years. He had to discover almost everything from scratch, but still, it's not that hard. Part of the reason is that he lost several years to the hole argument.

It's sort of off topic, but one area where Einstein's influence seems to persist is in the typical presentation of the ideas he originated. Look at the lede of our article on general relativity: "General relativity [...] is [a] geometric theory of gravitation [...] the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present", etc. Now look at the lede of the article on gauge theory: "[A] gauge theory is a type of field theory in which the Lagrangian is invariant under a continuous group of local transformations. [...] The transformations between possible gauges [...] form a Lie group which is referred to as the symmetry group or the gauge group of the theory. Associated with any Lie group is the Lie algebra of group generators", etc. This despite the fact that gauge theory has a geometric character very similar to general relativity, and you can just as well describe general relativity in this relatively impenetrable way. You even frequently hear people saying explicitly that gravity is not like the other forces. So the answer to "how long would it have taken the general public to figure out that relativistic gravitation is about the geometry of spacetime, if not for Einstein?" may be "at least 108 years, if not forever".

Likewise I'm sure no one would ever say that special relativity is based on two postulates if not for Einstein, I doubt anyone would ever introduce it using trains, and the awful word "observer" might never have entered physics (though that's not really Einstein's fault—he used it in its ordinary vernacular sense, not the weird technical meaning that it somehow acquired later). On the other hand you don't hear much about his "general principle of relativity" these days. -- BenRG (talk) 19:25, 21 February 2013 (UTC)

Wow, Ben, you grade hard. Remember that Einstein, with (in his own estimation) limited mathematical talent, had to learn differential geometry from scratch — a challenging course for mathematics doctoral students, and at the time, far less neat and polished (and influenced by GR specifically) than it is today. --Trovatore (talk) 08:52, 22 February 2013 (UTC)

But he had the right idea from the beginning with the equivalence principle and I think he was working like a dog that whole time. And he was Einstein. Oh well, history proves you right.

It's actually fairly surprising that real mathematicians didn't work all of this out a half century earlier. Sylvester understood the geometric meaning of positive-definite quadratic forms and proved his law of inertia in 1852, but it doesn't seem to have occurred to him (or anyone else) that there might be a more general kind of geometry corresponding to general quadratic forms. If it had been discovered, it's plausible that someone would have thought of applying it to physics, especially after Maxwell invented relativistic field theory in 1861. And Riemann, who died in 1866, actually tried to construct a geometric theory of gravity at the end of his life, and surely would have succeeded if he'd had the spacetime concept.

Quantum mechanics, on the other hand, I don't see how anyone could have anticipated. -- BenRG (talk) 04:13, 23 February 2013 (UTC)

Quantum mechanics certainly filled a HUGE hole in the existing theory, though. Someone had to solve the problems where experimental data didn't fit classical explanations, the Ultraviolet catastrophe established the clear need for a new theory. --Jayron32 04:20, 23 February 2013 (UTC)

This kind of thing is hard to prove conclusively - but if we look back to an era when new theories were not spread across the scientific community in a matter of days, there are many cases of similar breakthroughs being made completely independently. Calculus is a great example of that. Evolution is another. It seems that some level of 'background' math/science/data has to happen before it's possible for the next great thing to pop into someone's head - but once it does, someone will pick up on it in a matter of a few years. SteveBaker (talk) 20:14, 21 February 2013 (UTC)

Maxwell should have invented it. 23:04, 21 February 2013 (UTC) — Preceding unsigned comment added by Count Iblis (talk • contribs)

And had Faraday been a more accomplished mathematician, he should have beat Maxwell to explaining electromagnetism. So...Faraday should have invented it? Someguy1221 (talk) 09:22, 22 February 2013 (UTC)

Perhaps, but I think this was more about having the mind of a physicist than being extremely talented in math. So, I don't think David Hilbert or Poincare had the right stuff do develop even special relativity, despite Hilbert being able to look at a mathematical problem Einstein was struggeling with while working on general relativity and instantly finding the solution in his head (he didn't tell Einstein), while Einstein needed many more months of hard work to find it. Had someone like Einstein, Newton or Dirac lived in the early 1800s then I think special and general relativity would have been developed almost a century earlier. But you may not have gotten that result with some exceptional mathematician in that position (e.g. Euler, Gauss, von Neumann, Hilbert, Godel, etc. etc.). Count Iblis (talk) 12:23, 22 February 2013 (UTC)

I'm surprised no one has linked to these two articles: Multiple discovery and List of multiple discoveries. Ariel. (talk) 20:12, 22 February 2013 (UTC)

Average energy obtained for hydrolysis of a glycosidic bond and for a peptide bond

Anyone have a book or table which tells them what is the average energy obtained from hydrolysis of a glycosidic bond and also of a peptide bond? Many thanks, Kinkreet^{~♥moshi moshi♥~} 09:22, 21 February 2013 (UTC)

The change in enthalpy is often close to zero; entropic considerations will vary widely depending on the amount of water (free solution or crowded gel-like cell cytoplasm?), amount of product/starting material and steric considerations. 70.33.147.106 (talk) 02:28, 22 February 2013 (UTC)

pupae

Pupae found in Johannesburg Feb 2013

In Johannesburg, South Africa where i am currently on a holiday, the following pictures show what has been found on numerous walls. The pupae measure from 0.5 cm to 1 cm in length. I have been advised that a small black worm emerges from these and then will proceed to crawl up the wall pulling the shell or the pupae case with it. These are found usually at about head height or between 1.5 to 2 meters above the floor. I would please like your help in determining what this is. Thank you — Preceding unsigned comment added by Antonce (talk • contribs) 09:35, 21 February 2013 (UTC)

You will first need to upload the picture onto Wikipedia, or more preferably upload it somewhere else and provide a link to it. Simplying giving the pathway to the file on your computer does not actually allow us to see the picture. Kinkreet^{~♥moshi moshi♥~} 09:38, 21 February 2013 (UTC)

If what emerges is a larva, then this is an egg, not technically a pupa. See this similar thread on a South American bug. μηδείς (talk) 15:46, 21 February 2013 (UTC)

If what emerges is a larva then it is inside a larval case, as found commonly with the large number of bagworm moths and caddisflys among others. There are many species of bagworm moths in South Africa, see here and I suggest it is one of those possibly seeking a site to pupate. Richard Avery (talk) 08:16, 22 February 2013 (UTC)

Materials that are compatible with the use and storage of chlorine pentafluoride, perchloryl fluoride, monomethylhydrazine, and diethylenetriamine?

Materials that are compatible with the use and storage of chlorine pentafluoride, perchloryl fluoride, monomethylhydrazine, and diethylenetriamine? — Preceding unsigned comment added by 192.248.248.55 (talk) 21:27, 21 February 2013 (UTC)

Building a rocket are we? Those are without exception very nasty chemicals without many uses, I don't think this is one the RefDesk should field. Fgf10 (talk) 22:00, 21 February 2013 (UTC)

Sorry, but Wikipedia is not censored. We do have a policy of not answering questions that require medical diagnosis or legal advice, but questions about nasty chemicals are allowed. (It may very well be that they shouldn't be allowed, but the way to do that is to change the policy).

Wikipedia isn't censored, but please note that this is information not advice. In other words, if someone tells you that a substance is compatible with them, that does not mean that when you actually order something made of it off the Internet it won't have a contaminant, grease coating, etc. that would react 'vigorously' and cause a bad day. To quote Willy Wonka where our co-contributors are concerned, "No, no. I won't hold you responsible..." Wnt (talk) 00:29, 23 February 2013 (UTC)

192.248.248.55, start with reading the following articles (be sure to read the articles they link to as well): chlorine pentafluoride, perchloryl fluoride, monomethylhydrazine, and diethylenetriamine.

Then read Explosive material, Chemical explosive, Explosives safety, and Blast wave. Twice. Then decide on a safer propellant. One of the chemicals you list is a toxic gas, and one small leak will burn your skin and completely destroy your lungs. Not a nice way to die. --Guy Macon (talk) 04:17, 22 February 2013 (UTC)

According to John D. Clark, a good pair of running shoes should be compatible with all of them. --Carnildo (talk) 03:36, 23 February 2013 (UTC)

February 22

What is the velocity of a body on a circular path?

Consider a body moving on a circular path, its direction of velocity is changing at every instant of time. Therefore, the velocity of the body is also changing at every instant, so, its an accelerated motion. Suppose, the body is moving on the circular path with a speed of 50km/hr. Then, what is the velocity of the body? - 50km/hr or zero. Don't forget the body is changing direction at every instant. Technologous (talk) 01:45, 22 February 2013 (UTC)

Welcome to the Wikipedia Reference Desk. Your question appears to be a homework question. I apologize if this is a misinterpretation, but it is our aim here not to do people's homework for them, but to merely aid them in doing it themselves. Letting someone else do your homework does not help you learn nearly as much as doing it yourself. Please attempt to solve the problem or answer the question yourself first. If you need help with a specific part of your homework, feel free to tell us where you are stuck and ask for help. If you need help grasping the concept of a problem, by all means let us know. Someguy1221 (talk) 01:51, 22 February 2013 (UTC)

I don't see this so much as a homework request as a philosophical misunderstanding. Velocity is a Euclidean vector, which is to say that it has a directional component as well as a magnitude. As you've noted, for the provided circumstance, the magnitude of the velocity (the speed) is 50 km/hr. At any given point in time, the direction is along a line tangent to the orbital path, oriented in the direction of travel. It's well-defined and is not at any point in the orbit zero, but neither is it accurate to describe the velocity as simply "50 km/hr" -- that's only a speed, because it lacks the direction. — Lomn 02:05, 22 February 2013 (UTC)

This is not my homework, you think this as my homework question because its answer is very simple. I asked this simple question because I was confused what would be the velocity of the body when it is changing direction continuously. Should we consider the direction while talking about velocity? Again, what would be the velocity of the body - 50km/hr or zero? Technologous (talk) 02:20, 22 February 2013 (UTC)

Neither. Velocity is defined as a Euclidean vector, and accurately stating it requires both a magnitude and a direction. 50km/hr is the magnitude of the velocity, as Lomn stated. The direction is changing at all times, but at any given moment is tangent to the circular path at the position of the body. Someguy1221 (talk) 02:25, 22 February 2013 (UTC)

It's usually simpler to just think of that as a constant rotational velocity. StuRat (talk) 04:56, 22 February 2013 (UTC)

If a body is moving then it has a nonzero velocity. The net velocity over time may be zero, of course. The object has a zero velocity in the rotating frame of reference of someone riding on it who perceives it as stationary. Wnt (talk) 07:17, 22 February 2013 (UTC)

See Circular motion, this explanation, this simpler explanation with an animation. 86.163.209.18 (talk) 13:28, 22 February 2013 (UTC)

It occurs to me that you might be looking at average velocity. The average velocity of the object over a whole rotation will indeed be zero, in that the displacement ends up being zero. However, usually we find it more useful to talk about instantaneous velocity [3] [4] [5] for this sort of thing. If you haven't done any calculus yet, or haven't drawn any graphs showing velocity changing with time, you might find this confusing. Do keep thinking about it, because it sounds like your class is about to cover this in more depth. 86.163.209.18 (talk) 14:01, 22 February 2013 (UTC)

how different are the different species of manatees?

Seems like there are 3 species, but not good breakdown of differences.

• are ranges at all touching? (do animals cross ocean to Africa or do West Indian ones come in contact with Amazon ones?

• physical differention?

• can they cross breed?

• any cpntroversy if the are really differnt species versus subspecies?

Fat burning zone

I've heard doing cardiovascular exercises at 65% of maximum heart rate burns most amount of fat. But there are lot of sources in google search saying fat burning zone is a myth. I was reading this source (page 2) which says fat oxidation is highest when exercising at 65% of MHR. It further claims during 85% of MHR exercise intensity, fat oxidation falls. On the other hand, this source claims during 65% of MHR intensity, a higher percentage of fat is burned, but at higher intensity, the total fat calories burned is higher. This contradicts the claim in the first source. Which claim is true? --PlanetEditor (talk) 04:43, 22 February 2013 (UTC)

In my opinion, any such claims that are not accompanied by references to peer-reviewed papers supporting the claims should be ignored. If they had the science, they would tell us about it. --Guy Macon (talk) 09:37, 22 February 2013 (UTC)

Conditional Probability

I'm learning about probability in my statistics course, and I've learned that P(A|B) = P(A and B) / P(B). Similarly, P(B|A) = P(A and B) / P(A). Now, on a Venn Diagram, P(A|B) would be represented by the area that is within both circles as would P(B|A). However, if P(A) ≠ P(B) then P(A|B) ≠ P(B|A). If P(A|B) and P(B|A) represent the same area on a Venn Diagram, how can they ever be unequal? Have I understood some part here incorrectly? Lord Arador (talk) 04:51, 22 February 2013 (UTC)

I suggest you ask this Q at the Math Desk. StuRat (talk) 04:53, 22 February 2013 (UTC)

You have understood incorrectly. The area on the Venn Diagram that is part of both circles represents P(A and B). P(A|B) and P(B|A), on the other hand, are probabilities within a different set of possible outcomes, namely B and A, respectively. Someguy1221 (talk) 05:21, 22 February 2013 (UTC)

To expand on this: P(A+B) is the probability of being in the overlap of the two circles, given that you are somewhere in the whole diagram. P(A|B) is the probability of being in the overlap, given that you already know you are in circle B. P(B|A) is the probability of being in the overlap, given that you already know you are in circle A. If circles A and B are not the same size, then these probabilities will not be the same. 86.163.209.18 (talk) 09:48, 22 February 2013 (UTC)

See the Venn diagram in our article Conditional probability. P(A|B2) = P(A and B2) / P(B2) is numerically 0.12 / (0.12+0.04) = .12/.16 = 3/4. But P(B2|A) = P(A and B2) / P(A) is numerically 0.12 / (0.10+0.30+0.12) = .12/.52 = 3/13 < 1/4. Duoduoduo (talk) 17:51, 22 February 2013 (UTC)

Thanks so much, this clears it up perfectly. I also apologize for posting at the wrong desk.Lord Arador (talk) 22:50, 22 February 2013 (UTC)

Alternate wheel arrangement

I've seem some carts, for laundry and such, which have wheels in an unusual diamond arrangement. Here's a top view:

+------------------+
|        □□        |
|                  |
|□□              □□|
|                  |
|        □□        |
+------------------+

1) What is the name of this arrangement ?

2) Do we have an article on it ?

3) What are the relative advantages and disadvantages of this arrangement ? StuRat (talk) 05:13, 22 February 2013 (UTC)

This site[6] might help. Unfortunately it doesn't name the arrangement. Dncsky (talk) 05:40, 22 February 2013 (UTC)

The website provided by Dncsky has the key to it but doesn't spell it out. The key is that when this layout is used, all 4 wheels are fixed, i.e., are not swivel castors, and the two centre wheels are installed slightly proud. This means that the cart sits with weight on the 2 centre wheels and only one end wheel - the other end wheel is clear of the floor by 5 to 10 mm. By applying minimal force, you can rock the cart transfer weight to the other end.

Disadvantage: As weight is only on 2 wheels when turning, the load carrying capacity is reduced by 50% compared to other 4-wheel arrangements.

Advantage: By pushing, the cart moves reliably in a straight line, but by a bit of up/down force you can get the weight off both ends simulataneously and easily turn the cart. Other layouts that permit turning will not stably follow a straight line when pushed along.

Advantage: Because it is symetrical, the cart is equally manuverable pushing or pulling from either end.

You most often see this layouts on carts used by the stocking staff in supermarkets, where a) the aisles are narrow and long, b) there's not much room for manuevering, and c) the weight carried is not great, due to retail packaging.

Wickwack 58.170.140.138 (talk) 06:42, 22 February 2013 (UTC)

Another aspect: Look at each of the wheel arrangements an imagine a very heavy object placed in the corner. Some configurations will tip, others will not. --Guy Macon (talk) 06:54, 22 February 2013 (UTC)

Having used such carts in a hospital laundry, I found that they are able to negotiate certain small obstructions a lot easier than a conventional shopping trolley arrangement. Consider a sliding door that runs on a track. A shopping cart needs to be "forced" over the rail by rocking it side to side or just plain brute strength. With the above wheel arrangement, the leading wheel can be lifted off the ground simply by using your body weight to push the trailing wheel down, and so pass over the obstruction. Once the center axis wheels reach the obstruction, it is a lot easier "lift" the cart over the obstruction. It is not necessary to lift the center axis wheels off the ground. The leading wheel will take most of the weight - almost like a wheelbarrow. 196.214.78.114 (talk) 09:19, 22 February 2013 (UTC)

I once used a cart like that with large enough wheels that the deck cleared a standard curb. This made it somewhat easier to maneuver. --Guy Macon (talk) 09:34, 22 February 2013 (UTC)

Also, over time the corner arrangement will tend to cause the cart to sag substantially in the center, whereas the diamond configuration will not (as much).165.212.189.187 (talk) 16:24, 22 February 2013 (UTC)

regarding 3: You can roll it straight stable and easy with middle pair fixed wheels and alike easy turn it on the spot, if front and back wheel can turn. You cannot get the same resultcombination with classic four wheel layout in any combination of fixed or turnable wheels. --Kharon2 (talk) 23:27, 23 February 2013 (UTC)

However, with front and back wheels turnable (castors) it does not inherently run in a straight line when pushed, and so requires a varying lateral steering effort, which may be quite large if the floor is not dead flat. The standard supermarket style trolley, with all 4 wheels fixed, requires no lateral steering effort to run in a straight line, and is thus easier to use, particularly in supermarket aisles, which generally are pretty narrow. Wickwack 121.215.50.17 (talk) 01:41, 24 February 2013 (UTC)

That is true for the common supermarket trolley indeed but the questioned example was a cart for laundry. Imagine your efford pushing that supermarket trolley around a corner if its loaded with 500 Kg to understand the difference. --Kharon2 (talk) 10:49, 24 February 2013 (UTC)

Thanks, all. So we don't have an article on this ? StuRat (talk) 03:57, 25 February 2013 (UTC)

No, unusual wheel arrangements are to special to fulfill minimum relevance for articles. --Kharon2 (talk) 21:35, 25 February 2013 (UTC)

Blast wave

How can I calculate the minimum "safe" distance from an explosion involving a known number of carloads of TNT? Thanks in advance! 24.23.196.85 (talk) 06:39, 22 February 2013 (UTC)

No distance is "safe" - law enforcement has already seen your post and if anything happens, you're pretty easy to track. 86.101.32.82 (talk) 10:02, 22 February 2013 (UTC)

I'd look for a military safety manual and find the safe distance in peacetime for civilians or unprotected troops. Some googling found me this www.af.mil/shared/media/epubs/AFMAN91-201.pdf from the Air Force. There are probably OSHA standards or something similar for blasting work but I doubt that they include safe distances for those amounts of explosives. There might be regulations concerning the storage of large amounts of explosives, but I don't know where. Sjö (talk) 12:44, 22 February 2013 (UTC)

I don't think there is any one "safe" number you can calculate. For example if the explosion kicked up a rock, that rock can fly very very far, much farther than the blast wave itself. If you are able to calculate the size of the pressure wave then I can tell you that 5 PSI overpressure is normally considered the tipping point for structure damage. People can tolerate more than that - but are those people near a building? Other factors like that make it impossible to answer this question. For example, how high is the explosion off the ground? Does it have a surface behind it to reflect the blast wave? Ariel. (talk) 20:09, 22 February 2013 (UTC)

Just FYI, I'm not trying to blow anything up -- it's for one of the scenes in my WW2 thriller, where the good guys have to blow up a German ammo train at the station, but they specifically DON'T want to kill the signalman, so they tie him up and drag him away (normal Maquis SOP for this situation). So the reason why I'm asking is so I can figure out exactly how far they have to drag him. 24.23.196.85 (talk) 02:34, 23 February 2013 (UTC)

I'd drag him to the nearest ditch or other really substantial obstacle. You can survive surprisingly close to ground zero of an explosion if you've got something to shadow you from the shock wave and debris. --Carnildo (talk) 03:42, 23 February 2013 (UTC)

Good idea, Carnildo! Are irrigation ditches plentiful in the bocage? 24.23.196.85 (talk) 21:38, 23 February 2013 (UTC)

No, there aren't generally aren't irrigations ditches in France, the climate doesn't require them. There are drainage ditches instead, which are exactly the same ditches, running the other way. :p--Argarthar (talk) 02:02, 24 February 2013 (UTC)

Very well then, I'll drag him to a drainage ditch (preferably immediately behind a hedgerow -- the Normandy hedgerows can also provide limited protection from the blast). Thanks! 24.23.196.85 (talk) 00:29, 25 February 2013 (UTC)

Checked out the link that Sjo provided; that Air Force manual actually provides some very good methods for calculating the safe distance. Based on that, it looks like my "part-time commandos" will have to drag that signalman the better part of a country mile to ensure his safety. Thanks, Sjo! 24.23.196.85 (talk) 03:19, 23 February 2013 (UTC)

In which case, maybe they should just toss him in the back of their truck and drive away, and then put him out by the side of the road for the field-greys to find once they're at a safe distance? 24.23.196.85 (talk) 03:27, 23 February 2013 (UTC)

It would probably be wise to preface questions like this with "this is for a novel about WW2". That way, nobody will waste time informing you that what you are discussing is dangerous, and as a bonus answers will not assume technology that wasn't available before 1950. --Guy Macon (talk) 07:02, 23 February 2013 (UTC)

Yeah, I should've done that right off the bat. 24.23.196.85 (talk) 21:40, 23 February 2013 (UTC)

Time period of two identical pendulum

There are two simple pendulums and both are identical i.e., their strings are of same length (4 meters) and their bobs have same mass (m). One is kept in water and the other is kept in normal surrounding i.e., in air. Both pendulums are on the earth surface. What would be the ratio of time period of the two pendulums? 106.209.211.104 (talk) 10:20, 22 February 2013 (UTC)

I think it depends on two factors; first the density of the bob (because its effective weight is reduced according to the mass of water it replaces) and second the aerodynamics if water friction must be taken into account. - Lindert (talk) 10:29, 22 February 2013 (UTC)

Conflict - the period of a pendulum is independent of the mass of the bob. 196.214.78.114 (talk) 10:58, 22 February 2013 (UTC)

The period is normally independent of the mass only because the (effective) gravitational force is proportional to the mass of the bob. When the bob is immersed in water, that is no longer true. The period of a pendulum depends on the gravitational acceleration, and the latter changes when objects are immersed. For example, if the bob has a density equal to that of the water, it would not swing at all. - Lindert (talk) 11:57, 22 February 2013 (UTC)

The gravitational force doesn't change. Rather, there are new forces at play that didn't exist before: buoyancy and water resistance. Actually, buoyancy and air resistance exist in air, but their effect is so small that they can usually be neglected. --140.180.254.250 (talk) 19:21, 22 February 2013 (UTC)

I don't think you could come up with a "ratio of time period" in the above scenario. The friction on the pendulum in water would bring it to a standstill rather quickly - it would oscillate with a significantly shorter period each time. Remember that "The simple gravity pendulum is an idealized mathematical model of a pendulum." (see the wikipedia article). In real life one would strive for a frictionless environment to achieve a constant period - like in a pendulum clock - which uses a wound spring to offset friction and keep the pendulum going. 196.214.78.114 (talk) 10:52, 22 February 2013 (UTC)

I guess it depends on the size of their bobs too i.e. they would have different Reynolds numbers. Sean.hoyland - talk 15:25, 22 February 2013 (UTC)

It might be interesting to consider this question in superfluid liquid helium - apparently a torsion pendulum was used to prove ultra low viscosity in the first place, though I'm not so sure about pendulum bobs. Would certainly be funny to see. Wnt (talk) 23:54, 22 February 2013 (UTC)

Buoyancy and friction require knowing volume and cross-sectional area, respectively. For teensy bobs with tiny periods, the pendulums would swing exactly the same. Interesting question though. SamuelRiv (talk) 17:09, 24 February 2013 (UTC)

Matter wave diffraction

Suppose you fire electrons through a slit onto a screen, so it causes a pattern along the screen. If you have the probability distribution over the momentum for an electron (something that can be deduced from a given wave function) how do you then calculate the distribution of the electrons impacting on the screen? — Preceding unsigned comment added by 150.203.115.98 (talk) 13:27, 22 February 2013 (UTC)

Where did the mercury in coal come from?

Howcome coal contains mercury, selenium, and arsenic in the first place? is it that the nature when it formed were polluted with these substances? Then those plants and aninmal ought to have a higher tolerance for this? Electron9 (talk) 14:18, 22 February 2013 (UTC)

Firstly, look at the amounts involved: According to [7], burning coal in the US releases 50 tons of mercury per year. This source [8] says that the US mines more than a billion tons of coal per year - and burns 90% of it (the rest is exported). So the amount of mercury in coal is roughly 50 parts per billion. Coal forms from rotting plant material that turns into peat. According to this [9] about half of the mass of a tree is carbon - and that's pretty much all there is in coal - so the plants from which coal is formed had to have about 25 parts per billion of mercury in them. According to mercury poisoning the USDA limit for mercury in food is one part per million - so 500 parts per billion. Hence, we may deduce that the plants that formed coal had about 5% of the USDA limit...the animals of that era had no risk from mercury poisoning. SteveBaker (talk) 14:34, 22 February 2013 (UTC)

I meant more, was the overall mercury level in the environment higher when it formed than it is now? and in particular how high then and now? Electron9 (talk) 15:34, 22 February 2013 (UTC)

I searched google scholar for /paleoclimate mercury/. One of the top hits was this paper that appeared in Science in 1999: "Mercury in a Spanish Peat Bog: Archive of Climate Change and Atmospheric Metal Deposition"-- you can read the abstract (and whole article if you can get access through a library) here [10]. Anyway, I haven't read the whole article, but this sentence- "Anthropogenic mercury has dominated the deposition record since the Islamic period (8th to 11th centuries A.D.). "- leads me to believe that mercury levels were generally lower prior to the anthropocene. The paper only analyses samples up to 4k years before present, which is too recent for coal from the carboniferous, but there are probably papers in the references (or that cite this one) that go back further in time. SemanticMantis (talk) 17:12, 22 February 2013 (UTC)

The key thing to remember here is that the coal deposits we burn weren't laid down in the few years it has taken us to run through them. They represent the accumulated debris of millions of years, and whatever survives the trip up the flue ... comes back down to haunt us faster than it can wash out to sea. Wnt (talk) 00:01, 23 February 2013 (UTC)

BCNT - what's the deal with 11B?

Boron neutron capture therapy works by ¹⁰B absorbing a neutron to give ¹¹B that decays, releasing destructive energy in its vicinity, killing cancerous cells. At least I think that's how it works. The catch is that ¹¹B is apparently rather stable nucleus, comprising 80% of boron in nature. So is the initially formed ¹¹B derived from ¹⁰B some sort of labile excited state?--Smokefoot (talk) 14:23, 22 February 2013 (UTC)

Apparently a metastable state. I can't find a ref for a distinct ^11mB nuclear isomer, but I do see some comments that there is some excess energy from the ¹⁰B + n → ¹¹B reaction. I don't know if it's just momentum from the collision that winds up being dissipated by fission rather than some other emission or increased vibration? How does the actual energy balance work?--is there a change of nuclear mass defect to indicate that that some of the incident neutron's mass winds up as available energy? DMacks (talk) 16:36, 22 February 2013 (UTC)

Wouldn't you expect the newly created ¹¹B to emit a gamma ray? μηδείς (talk) 15:27, 23 February 2013 (UTC)

An alloy that expands on heating

My question is on alloys.

Name an alloy which expands on heating.

Thanks for your help Ssmagic (talk) 16:58, 22 February 2013 (UTC)

Almost every alloy expands on heating. Looie496 (talk) 17:01, 22 February 2013 (UTC)

Are there any at all which don't? Not including unverified claims such as those for YbGaGe--Gilderien Chat|List of good deeds 19:59, 22 February 2013 (UTC)

Invar has a coefficient of thermal expansion close to zero. 24.23.196.85 (talk) 03:13, 23 February 2013 (UTC)

There are a number of materials that have a negative thermal expansion coefficient over a temperature range. Water, for example, contracts when heated from 0C to 4C, and zirconium tungstate contracts over almost its entire solid temperature range. --Carnildo (talk) 03:52, 23 February 2013 (UTC)

Expanding alloys

My question is on alloys

Name an alloy that expands on coolingSsmagic (talk) 17:02, 22 February 2013 (UTC)

Sorry, my earlier question was incorrect

Thanks

We have an article on this, actually. You can check negative thermal expansion for some answers. Livewireo (talk) 17:54, 22 February 2013 (UTC)

This sounds like a homework question. Woods metal and Field's metal are two very common alloys that would both fit the bill.--Aspro (talk) 17:58, 22 February 2013 (UTC)

See an example of it used for Movable type#Type-founding where the expansion of the metal makes strong sharp edges in the letters. It's an allow of lead tin and antimony. Ariel. (talk) 19:22, 22 February 2013 (UTC)

Search for some bismuth alloy. OsmanRF34 (talk) 19:46, 22 February 2013 (UTC)

A sealed ecosystem that thrives for 40 years???

All around the web this story is going strong, here's one example [11]. I updated the article about Ecosphere_(aquarium) and noted that according to the article Every manmade closed system inevitably degrades. Has Latimer allegations been tested? Where can this conundrum be further researched? Zarnivop (talk) 18:21, 22 February 2013 (UTC)

Well, the bottle isn't a closed ecosystem in that it relies on outside light. I also wonder about the carbon dioxide being released by rotting vegetation. I wouldn't expect 100% of the plant's CO₂ needs to be fulfilled by that mechanism. I would think you'd need animals to convert free oxygen back to carbon dioxide. Perhaps microscopic animals are present. StuRat (talk) 19:11, 22 February 2013 (UTC)

It's a matter of time scale. 40 years may seem like a long time, but it really isn't, insofar as the nitrogen cycle, water cycle, and carbon cycle of that bottle are concerned. Predicting exactly what will cause it to fail/die is difficult, but "inevitably degrades" is true of this bottle too. SemanticMantis (talk) 19:39, 22 February 2013 (UTC)

If you wait long enough, the earth will "inevitably degrade" when the sun becomes a red giant. Many of our our pages in this area are a mess. I would encourage anyone who has the time and interest to gather some citations and rewrite portions of the articles

If you look at http://www.synergeticpress.com/wp-content/uploads/2012/04/Bioscience-1993-using-a-closed-system-to-study-the-Earth.pdf ("Previous research in closed ecological systems" section) some systems are talked about that were closed in 1968 and were still functional as of the 1993 date that the paper was published (25+ years). As far as I know, there is no upper limit to how long a Winogradsky column can operate

Articles that need improvements and/or sections about how long they can last:
Closed ecological system
Controlled Ecological Life Support System
Spome
Wardian case
Winogradsky column
Eden Project
Biosphere 2
BIOS-3
MELiSSA
Biosphere
--Guy Macon (talk) 20:20, 22 February 2013 (UTC)

My feeling is that the odds of degradation also should depend on the number and type of species present - specifically, any closed system should be subject to Island biogeography. For example, if you look at that plot in the article for the number of lizards on Caribbean islands, you can predict an intercept somewhere around the size of a football field to keep even one species alive. Below that and sooner or later you'll have a tragic mishap and end up with all males or something. (I really should review the theory of single species metapopulation dynamics - there are lots of studies like [12] based on a 1970 theory by Richard Levins that do real-life games of life to see what happens) A bottle garden with a plant is harder to picture becoming unstable - nonetheless, there's no reason to suppose that the plant's lifespan will be infinite, and it is possible that when the whole plant decays that the parameters inside the bottle will go outside the range that any potential seedling can tolerate. Wnt (talk) 23:04, 22 February 2013 (UTC)

I gather from this discussion that the statement as presented in the article is false. I edited the article. I also try to convince my wife to try spiderwort sphere, and I may get back to you in 20 years or so. Zarnivop (talk) 16:00, 23 February 2013 (UTC)

ips cells

How do ips cells work? Clover345 (talk) 19:19, 22 February 2013 (UTC)

You might find the Induced pluripotent stem cell article to be helpful. Red Act (talk) 19:35, 22 February 2013 (UTC)

does pwm (pulse width modulation) led's hurt eyes?

does applying pwm to 'dim' LED's in fact hurt eyes, as they are on full-blast, albeit a shorter period of time? 86.101.32.82 (talk) 21:55, 22 February 2013 (UTC)

It would all depend on how the actual light level changes. If they go from black to full brightness and back, then it might cause a problem. However, I doubt if they do that. More likely, they just get a bit dimmer and brighter going through the cycle. Also, even if it did go completely black in the cycle, if it did it quickly enough (say 60 or 70 times a second), our eyes would only see the average, not flickering, so it would be OK. StuRat (talk) 22:03, 22 February 2013 (UTC)

This is what I mean - why would our eyes "see the average"? I mean, if you imagine a laser that is bright enough to burn a retina, is it "ok" as long as it's only on for 1/1000th of a second? What I mean is that there must still be a difference between every photon that is going to come through in 1 ms coming through at an even pace throughout that ms, or instantaneously at the start of that ms...don't you think? 86.101.32.82 (talk) 23:15, 22 February 2013 (UTC)

Well, chemical reactions take time, including those which would cause damage to the retina. For example, if the LED is going to raise the local temperature to a point where the retina will be burnt, it will take some time for the temperature to build to that point. Now, if the photons were coming in quickly enough, that time period might be extremely low. However, the level of photons coming off an LED probably isn't enough for that. So, this gives the retina a chance to cool down when it is switched off. This continual heating and cooling results in a temp close to the average.

A useful analogy here might be the Earth's day and night cycles. If the Earth didn't rotate relative to the Sun, then the side facing the Sun would be too hot for us to live, and the dark side would be too cold for us to live. But, with it rotating, it starts to heat up during the day, then starts to cool down at night, never getting to the extremes it would reach if stationary. StuRat (talk) 23:23, 22 February 2013 (UTC)

It sounds from [13] like a pulse laser takes maybe 40x less energy to burn the retina if it produces "giant" pulses with a high peak power than "long" pulses with lower power, but I'm afraid I'm missing some of the context to understand it fully; in particular I missed how often these pulses were. Not really my thing - this was a top hit from a Google search; probably you can find better studies. Wnt (talk) 23:31, 22 February 2013 (UTC)

In the case of the laser, the total power is too low to cause damage, but because the laser beam is concentrated in a small area of the eye, the short pulse heats that area faster than the heat can dissipate, while the long pulse doesn't. --Carnildo (talk) 03:58, 23 February 2013 (UTC)

Humidity

A question asked earlier piqued my interest. Why would one humidify a room? I live in Ireland and can't really imagine what a "too dry environment" would feel like. — Preceding unsigned comment added by Stanstaple (talk • contribs) 23:25, 22 February 2013 (UTC)

In parts of the world where it gets very cold and dry in the winter (the American midwest, for example), humidity in the living space feels better than dryness. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:33, 22 February 2013 (UTC)

Your skin gets dry, starts to itch, and eventually cracks. The mucous membranes in your nose start to dry out, which is uncomfortable and makes you more susceptible to sinus infections. Very dry air is also bad for musical instruments. thx1138 (talk) 23:44, 22 February 2013 (UTC)

(See winter itch) Wnt (talk) 23:59, 22 February 2013 (UTC)

You might try half-an-hour of mouthbreathing in a restaurant's walk-in freezer. For the first time this winter I plugged in a humidifier last night. I have a sore-throat from the dryness. μηδείς (talk) 23:52, 22 February 2013 (UTC)

In my case I get chapped lips, which will crack open and bleed if I don't apply lip balm, and deep cracks in the callouses on the soles of my feet, which would also crack open and bleed, if I didn't apply bag balm to them each night. Fortunately, I have generally oily skin, so the rest is OK. On the plus side, I can just toss my grapefruit peels in the garbage, and they dry out, whereas they would get fuzzy if I tried that in (humid) summer. StuRat (talk) 00:01, 23 February 2013 (UTC)

Chapped lips - deep cracks in the callouses?!!! See your doctor. This is symptomatic of a lack of Vitamins A, B's , C, and E. Especially B2. Is your skin ever scaly etc.? By a rifle (no, not an AK 47 off off eBay but a good one) and hunt your own food. Eat everything, liver, kidneys, brain, lungs, etc., and let you wife fry up the gonads. Eat with freshly ground wholemeal bread. Vitamin deficiency gone -leaving you with lips that a moose would love to kiss and feet like Fred Astaire.--Aspro (talk) 01:01, 23 February 2013 (UTC)

I don't think so. This happens every winter. Due to the poor insulation in this house, the relative humidity inside stays around 20% all winter. I have a healthy diet and take vitamins supplements, and don't have any problems, so long as I remain properly em-balmed. StuRat (talk) 08:19, 24 February 2013 (UTC)

<peeve> calluses, no o. Callous is an adjective; callus is a noun. </peeve> --Trovatore (talk) 04:06, 23 February 2013 (UTC)

How callus of you to make that wise-crack. (Now don't you feel like a heel ?) :-) StuRat (talk) 08:14, 24 February 2013 (UTC)

I'm not sure about the relation of chapped lips to weather. I don't find much about it at all in PubMed; one paper that mentions it is a study in military recruits during the summer. I've seen the claim in textbooks and other sources citing old studies that there is a relationship between these and riboflavin levels (which causes angular cheilitis when deficient) [14]. Wnt (talk) 00:16, 23 February 2013 (UTC)

Low humidity definitely has an effect on my lips. I don't know why they have difficulty in determining this. Why wouldn't the extremely thin skin on the lips allow moisture to escape, and why wouldn't this rate of evaporation be dependent on the atmospheric humidity ? I've also noticed that salty foods make my lips chapped. StuRat (talk) 00:36, 23 February 2013 (UTC)

Well, personally I've never used a lip balm, including in Midwest winters; since I read about the vitamin issue as an undergrad I've taken any roughness at all as good reason to take a multivitamin - but in truth I do that very rarely. Wnt (talk) 08:06, 24 February 2013 (UTC)

I don't understand. You think chapped lips are only caused by vitamin deficiencies ? StuRat (talk) 08:14, 24 February 2013 (UTC)

I shouldn't say that. As I said above, I really don't know what causes them - there's not much literature to go by. All I'm saying is that I'm skeptical that they are the ordinary result of dry weather. Given the various stuff on the web about people so skeptical that lip balms are "addictive" that they actually have an urban legend about "ground glass" in them, I'm getting the feeling that people who use balms are not really reaching the root of the problem. Wnt (talk) 08:50, 24 February 2013 (UTC)

I suppose the root of the problem is that humidity is too low, but it's not very realistic to change that in some places (like outside), so treating the symptom with lip balm seems like the way to go. At $1 a tube, I find it solves the problem quite nicely. StuRat (talk) 04:05, 25 February 2013 (UTC)

I wonder if the poster comes from that south western part of Ireland where the only dry parts are were the Garda Síochána na hÉireann have convisicated all the poitín stills. Take a holidy in the Arab Emirates. 105 Fahrenheit in the shade, relative humidity 20% and you'll drink a gallon a water an hour... as the whole land is tea total – then you'll under stand dry--Aspro (talk) 00:31, 23 February 2013 (UTC)!

Here is a description of what living in low humidity is like: http://livingintok.wordpress.com/2011/12/14/the-dangers-of-low-humidity/ --Guy Macon (talk) 07:10, 23 February 2013 (UTC)

Because we in the British Isles are surrounded by water and most of our weather comes either from the Atlantic or the North Sea, it never gets really dry here. The humidity in both London and Dublin today is 81%. Alansplodge (talk) 12:28, 23 February 2013 (UTC)

Something else bad about low humidity is it's effect on wooden furniture, which can warp or crack. Applying oil to it helps, but you really need to keep the humidity up, too. (Technically it's the change in humidity which is the problem, so, if you grew a tree in low humidity, and always kept it's wood in low humidity, it would be OK.) StuRat (talk) 16:52, 23 February 2013 (UTC)

what's the essential difference between capacitor and a battery?

what's the essential difference between capacitor and a battery? What I mean is, with the recent "breakthrough" of the guy printing a dvd of carbon capacitor, like, could it be that it stores 1 kilowatt-hour on it and discharges it over 5 minutes? Meaning you just draw 12,000 watts for 5 minutes continuously? (The example could be reread at 1/10th rate: store 0.1 kw-hours on it, meaning drawing 1200 watts for 5 minutes conntuously). The reason I ask is that the NORMAL way i think of capacitor is "instant discharge" (<1 second). But aren't there slow discharge capacitors?

In this case what is the essential (ESSENTIAL) difference or line in the sand between a (slow-discharge) capacitor and a battery? The carbon printing on CD guy mentioned charging a capacitor briefly and then powering a led on it for a long-time? THen isn't it technically a battery and not a capacitor? Thanks. Also motor capacitor. Thanks. 86.101.32.82 (talk) 23:55, 22 February 2013 (UTC)

A capacitor stores electrons and/or holes without alteration, such as in the conduction band of a metal. There is no chemical reaction. A battery reacts to uptake those electrons into some compound or another, which means that more electrons can come in without being repelled by those already stored, until the amount of material practically available to react is used up. Wnt (talk) 00:19, 23 February 2013 (UTC)

Also, a capacitor stores a net charge. Batteries are electrically neutral. Batteries work by chemical reactions that maintain electrical neutrality, but work by moving electrons from areas of high potential to areas of low potential, and replacing the electrons so moved by moving ions into their place. See Galvanic cell for a nice, idealized picture of how all batteries basically operate. The key is, at no point and nowhere in the battery is there a net build up of charge (or indeed, anywhere in the circuit). In a capacitor, you have a net buildup of both positive and negative charges on the two poles, batteries don't work that way. --Jayron32 00:47, 23 February 2013 (UTC)

To avoid confusion: a capacitor has no net charge overall. It has 2 plates, one of which has a charge of Q, and the other has a charge of -Q. The two sum to 0. --140.180.254.250 (talk) 00:49, 23 February 2013 (UTC)

I remember reading an article about work on this sort of capacitor shortly after 9/11. One problem was they could discharge so fast they were essentially little bombs. μηδείς (talk) 00:51, 23 February 2013 (UTC)

That's true, 140, but there is a localized buildup of charge on each pole of the capacitor. No such localized buildup of charge exists in a battery, which from my point of view, is the essential difference. --Jayron32 01:12, 23 February 2013 (UTC)

The distinction between a capacitor and a battery can sometimes be blurry nowadays. In particular, one type of supercapacitor is a hybrid capacitor, which stores a charge both electrochemically and electrostatically. Red Act (talk) 02:02, 23 February 2013 (UTC)

Well, true, but the very existence of a hybrid category only highlights the clear distinctions in the traditional devices. --Jayron32 02:20, 23 February 2013 (UTC)

There are two fundamental differences between batteries and capacitors. 1) A battery works by storing electrochemical tension, and a capcitor works by electrostatic tension, as explained by some of the posts abeove, and 2) for a constant current load, a battery delivers essentially constant voltage over time until it reaches a point of exhaustion, whereupon the voltage drops rapidly; A capacitor with a constant current load delivers a voltage continously and linearly decreasing with time.

It is perfectly feasible (though not gnerally cost effective) to use a capacitor of sufficient size to run an electrical device (motor, radio, etc) however allmost all devices require a near constant voltage. A battery does this inherently, but a capacitor will require a power conversion circuit (switch mode power convertor) in order to do this.

Capacitors, because they work by electrostatic tension, inherently are precise devices if used for timing purposes. As batteries work by means of electrochemical tension, and chemical reactions are very much temperature dependent, The time to discharge in a battery is generally an innaccurate thing.

As mentioned above, you do get devices that differ to some extent from what I've listed above. For example, electrolytic capacitors do not have very accurate capacity, and some capacitor types show a degree of electrochemical storage.

Keit 120.145.29.214 (talk) 05:55, 23 February 2013 (UTC)

February 23

Identity involving partial derivatives

Ho, I want to check,

Given this relation: $dF=-SdT-PdV+\mu dN$,

is it true that ${\bigg (}{\frac {\partial S}{\partial V}}{\bigg )}_{T,N}={\bigg (}{\frac {\partial P}{\partial T}}{\bigg )}_{V,N}$?

I believe the answer is Yes and here is my reasoning:

The relation shows that $S=-{\frac {\partial F}{\partial T}}$ and $P=-{\frac {\partial F}{\partial V}}$,

so ${\bigg (}{\partial S \over {\partial V}}{\bigg )}_{T,N}{\stackrel {?}{=}}{\frac {\partial S}{\partial V}}=-{\frac {\partial ^{2}F}{\partial V\partial T}}{\stackrel {?}{=}}-{\frac {\partial ^{2}F}{\partial T\partial V}}={\partial P \over {\partial T}}{\stackrel {?}{=}}{\bigg (}{\partial P \over {\partial T}}{\bigg )}_{V,N}$

I have denoted with question marks steps in the reasoning that I am not sure are justified.

150.203.115.98 (talk) 02:11, 23 February 2013 (UTC)

Try Wikipedia:Reference desk/Mathematics instead. --Jayron32 02:21, 23 February 2013 (UTC)

The mathematics desk might be more relevant, but this is a thermodynamics question, so it's not out of place here.

To the OP: yes, that's true. In fact, it's one of the Maxwell relations. Your biggest source of confusion is that when taking partial derivatives, you should always specify what parameters you're holding constant. The relations you're using should actually be $S=-({\frac {\partial F}{\partial T}})_{V}$ and $P=-({\frac {\partial F}{\partial V}})_{T}$, where V and T are the quantities being held constant. If you differentiate the first equation with respect to V while holding T constant, and the second equation with respect to T while holding V constant, you'll get the same expression on the right-hand side. You're allowed to arbitrarily exchange variables in a partial second derivative because of symmetry of second derivatives, specifically Clairaut's theorem. --140.180.254.250 (talk) 03:25, 23 February 2013 (UTC)

Clairaut's theorem requires that the second partial derivatives be continuous. Is this always true, physically? Or, do we assume/approximate that it is true? Or, is the statement to prove true even if the second partial derivatives are not continuous?

So the true line of reasoning is

${\bigg (}{\partial S \over {\partial V}}{\bigg )}_{T,N}=-\left({\frac {\partial ^{2}F}{\partial V\partial T}}\right)_{N}=-\left({\frac {\partial ^{2}F}{\partial T\partial V}}\right)_{N}={\bigg (}{\partial P \over {\partial T}}{\bigg )}_{V,N}$

Using the equations you gave (but also holding N constant)

150.203.115.98 (talk) 05:58, 23 February 2013 (UTC)

The second derivatives of the potential are not always continues, of course. They are not in any second order phase transition, where Ehrenfest equations hold instead. Ruslik_Zero 19:03, 23 February 2013 (UTC)

What's a partial derivative? μηδείς (talk) 02:16, 24 February 2013 (UTC)

Non-calculus proof of E=mc^2

See [1], this non-calculus proof of E=mc^2. We know, F=m*a (where m is mass and a is acceleration) and it is given in the second equation of the linked article that F=m*c. How can this be possible? Since the SI unit of 'a' is m/s^2 while the SI unit of 'c'is m/s. According to me, this proof is wrong. I read the Wikipedia article Mass-energy equivalence, but the proof of E=mc^2 was very tough for me. Is there any other non-calculus (and other than dimensional analysis) proof of this equation? Thanks in advance. 106.216.105.77 (talk) 13:55, 23 February 2013 (UTC)

On the linked page, m in the second equation seems to represent the mass gained per unit time and so has units of kg/s rather than kg (so the units come out as kg m/s² on both sides). Similarly in the first equation E is energy gained per unit time and so has units of J/s (i.e. W) rather than simply J. Since the argument compares rate-of-change of energy with rate-of-change of mass, it is calculus (sort of) - but writing e.g. the rate-of-change of energy as E rather than dE / dt disguises this. --catslash (talk) 14:27, 23 February 2013 (UTC)

It's an elegant argument, but not mathematically stringent. It avoids a proper calculation of the limit, and simply assumes that speed is constant (which is not, strictly true). But it does provide a nice physical insight. --Stephan Schulz (talk) 14:50, 24 February 2013 (UTC)

Coal Fired Boilers - Briquettes

Hi

I was asked to get info on making briquettes with company waste such as coal dust mixed with saw dust or wood chips.

My question is, can the two substances be mixed together and processed together to make briquettes for coal fired boilers? If so what is the mix ratio to make it a cost saving and green project? Where can I get good drawings or advice?

Currently, the company is under pressure to reduce costs. All the coal dust and wood chips go a dump site and I don't agree with it.

This is items which can be recycled and will give cost savings on the long run.

Any solutions?

Regards

Gert de Ridder (Email address & phone nummer removed) — Preceding unsigned comment added by 105.226.163.32 (talk) 15:40, 23 February 2013 (UTC)

I have removed your personal details for security - Cucumber Mike (talk) 15:59, 23 February 2013 (UTC)

My understanding is that those things don't generate as much energy per unit volume as coal does, so I think you will need to find out how flexible your boilers are. This comes down more to what the boilers can handle than to what you can do with the waste. Looie496 (talk) 16:11, 23 February 2013 (UTC)

Note that the mass of the briquettes, and how well compressed they are, will determine how fast they burn, with low mass, uncompressed ones burning faster. So, if the mass of each is low enough, and/or they are uncompressed enough, they should provide sufficient heat. However, they won't last as long. You could just experiment with different sized briquettes to find the right form. How do you intend to compress them ? I wonder if a trash compactor would work. You might need to add some type of flammable binder. StuRat (talk) 16:59, 23 February 2013 (UTC)

Reusing industrial scrap wood for briquettes goes back at least to Henry Ford and the Model T (Kingsford (charcoal)). It is a well-understood process that should be well documented. Rmhermen (talk) 17:23, 23 February 2013 (UTC)

Also note that, while coal dust can be burned directly, you need special equipment to do so. Trying to burn it in a regular coal furnace may result in an explosion. (I know you're not planning on doing this, but feel the warning is needed, just in case.) Therefore, you should store the coal dust in a sealed container, far from an ignition source, prior to compacting it into briquettes. StuRat (talk) 17:43, 23 February 2013 (UTC)

• First Gert. Where are you located on this huge planet? Some governments give grants to instal equipment to make use of what otherwise is waste – so knowing you location would help. Second, what cheapskate company are you working for that doesn’t want to contract a consultant? To economically utilize waste as fuel, inevitable require some outlay in equipment. To make sure that this this expense is economically recoverable and viable, a good costing exercise is required -hence need for a qualified consultant (vis Wikipedia). It is better to use tried and tested technology, than to follow what you and your employer can glean from Wikipedia (although having said that, please pay me... ASPRO US$1000 for this consultation as it will save your company thousands more dollars further down the line) . For an example, this company builds bespoke waste processing plants [15]--Aspro (talk) 18:28, 23 February 2013 (UTC)

• For a small project like this, the consulting fee may not be recovered for years. (And if they don't have to pay Gert overtime, then working on this project won't cost them any extra, as opposed to the consultant.) StuRat (talk) 19:01, 23 February 2013 (UTC)

That is why I wanted to know where he was located. Some grants are available to pay a boffin to come in and say – don't shovel the coal-dust strait into the furnace as it will suffocate the fire bed. Mix it first with a little binder (say cellulose wall paper paste) and then throw the binded chunks in... Do you know how much little companies have to pay just for legal advice? Advice from popper recycling expert is cheap in comparison. If they think they can not afford to pay for that, then our poster should think seriously about working for another company that is still going to be in business this time next year. --Aspro (talk) 20:18, 23 February 2013 (UTC)

• ...and it will look great on his resume. --Guy Macon (talk) 19:19, 23 February 2013 (UTC)

No. it may look impressive on his résumé but at a job interview he may come across a just a lucky amateur – providing he even succeeds in fulfilling his current employers wishes.--Aspro (talk) 20:18, 23 February 2013 (UTC)

Selective Knockout

Hello. I want to demonstrate whether the heart is the only tissue that requires Mef2 for embryogenesis in mice. Do I knockout Mef2 in every tissue that expresses it except for the heart? If so, how can I go about that? Thanks in advance. --Mayfare (talk) 17:42, 23 February 2013 (UTC)

What approaches have you considered so far? You'll forgive me, but this sounds a bit like a homework problem.... TenOfAllTrades(talk) 19:05, 23 February 2013 (UTC)

Does sound a bit like homework - Let's stake out gene knockout, Mef2, promoter (genetics), enhancer (genetics) to begin with. Especially the Mef2 article because if you're knocking out all four you'll definitely be busy ... You may decide you find gene knockdown to be of interest. Wnt (talk) 20:18, 23 February 2013 (UTC)

Feeding of K-9 dogs

My friend told me that K-9 dogs (law enforcement dogs) are only fed once a day. And she said that this is done to keep them "hungry and fierce".

Is it true that K-9 dogs are only fed once a day? If so, is "keeping them hungry and fierce" really the reason? I don't see why you'd want that -- it doesn't sound like their job description.

Thanks. And I would be extra grateful if you cite to a source...

99.100.214.111 (talk) 22:35, 23 February 2013 (UTC)

The International Police K9 Conference doesn't say anything about the frequency of feeding, but it does say; "Working dogs use a lot of energy. To replace this, a large amount of high quality, nutritious dog food is required. These dogs should be offered all of the dog food they want, unless they begin to become overweight." So it doesn't seem to be recommended that they be kept hungry. However, The Glendale Police K-9 Unit says; "They normally eat once a day at the end of our shift. Feeding them before work will cause them to be sleepy, just like after we eat a big meal." I'm not sure if this is universal practice. Alansplodge (talk) 00:17, 24 February 2013 (UTC)

No, being fierce is NOT part of a police dog's job decription. Many police dogs are drug or explosives detection dogs - for this what is wanted is a well fed happy dog that just wants to enjoy the "game" of detection and please its' master. For apprehension dogs, the last thing you want is a fierce dog - want you want is a fit dog that obeys commands absolutely. The idea is to bring down and restrain the villain, not damage him. Police dogs are generally the larger and more intelligent breeds e.g., german sheperd. These large breeds only need a meal once a day, whether they are in police service or are just pets. This does not preclude giving them small treats now and then, which when done sparingly and at the right moments, gives you a better dog. Small breeds may need meals twice a day. See http://www.purina.co.uk/content/your-dog/feeding-your-dog/the-right-food-for-your-dog/feeding-your-adult-dog. Wickwack 121.215.50.17 (talk) 02:18, 24 February 2013 (UTC)

A childhood friend's father was a London police dog handler. The dog had a huge kennel in their back garden, which we used to play in along with the dog. I'm sure it could have easily taken off one of our arms or legs, but she was a model of tolerance. Alansplodge (talk) 09:29, 24 February 2013 (UTC)

That could be misleading though. One of the built-in virtues of large dogs is that they instinctively understand that children are human puppies. My own experience of large dogs is that they'll tolerate a lot of horseplay and rough handling from children that they will not tolerate from adults. They treat people they know differently to people they don't know too. Wickwack 60.228.244.46 (talk) 11:05, 24 February 2013 (UTC)

It's actually not abnormal to feed a dog once a day. We had a labrador cross that lived to almost 15 and we only ever fed her once a day, and she was never "hungry and fierce". Vespine (talk) 21:59, 24 February 2013 (UTC)

Is it possible that a supertanker or a cruise ship that is made of all woods, treenails and tar, can float for a long time?

Let's suppose people build a supertanker or a cruise ship that is made of all woods and tar? Can it float on water for a long time? Why can the ship float and why can't it float? 173.32.116.184 (talk) 23:35, 23 February 2013 (UTC)

The Influence of Iron in Ship Construction: 1660 to 1830 says that the bigger wooden ships were subject to flexing and twisting of the hull, causing the wooden joints to open. Early in the 19th century, iron bracing was being fitted to large wooden warships in order to stiffen them. The introduction of steam propulsion required an even more rigid hull. So when the technology to make an iron ship became available, HMS Warrior (1860) was the result. The biggest wooden ships were tiny compared to modern steel vessels; HMS Victory is 70 metres long, RMS Queen Mary 2 is 345 metres and the TI class supertanker is 380 metres. Alansplodge (talk) 23:56, 23 February 2013 (UTC)

To add a couple of specific examples - the oldest wooden ship still afloat is the USS Constitution (215 years), although she's undergone a great deal of maintenance in that time. The largest wooden ship ever was the Baron of Renfrew, which lasted for three months. The largest working wooden ship was the Great Republic, which lasted for 19 years. Tevildo (talk) 00:19, 24 February 2013 (UTC)

Good examples - Baron of Renfrew was 93 metres long and that was too much. Alansplodge (talk)

See also Treasure ship (Zheng He). Wnt (talk) 08:03, 24 February 2013 (UTC)

If you make an unballasted wooden ship (oddly, we don't appear to have an article on the general use of ballast in ships), it'll float no matter how much water it takes on (assuming it isn't carrying much cargo), but an unballasted ship also tends to be topheavy and at risk of capsizing. --Carnildo (talk) 05:39, 24 February 2013 (UTC)

In that case, it's also called a raft ;-) --Stephan Schulz (talk) 15:20, 24 February 2013 (UTC)

February 24

Crushing

I remember reading about the study of crumpling or crushing paper or another two-dimensional object. But we don't seem to have an article on this. Are there any sources giving information on how well paper crumpled within a cylinder can support a weight above it? Thanks. μηδείς (talk) 02:25, 24 February 2013 (UTC)

I don't quite understand. The more weight you place on it, the more it will compress, until it reaches a point where it can't compress much more (about the density of wood, I imagine). StuRat (talk) 08:08, 24 February 2013 (UTC)

The question has relevance and merit if the crumpled paper is used for packing fragile products. There's bound to be someone who has studied it from that angle - what density of object can be protected by crumpled paper packing. What is the optimum crumpling density for use in packing? Of course, whether or not this is what Medeis had in mind - who knows? Wickwack 60.228.244.46 (talk) 10:56, 24 February 2013 (UTC)

My interest is in the abstract. I remember reading about ten years ago that researchers had found some sort of fractal constant that would predict how much a two-dimensional sheet of material--say paper or aluminum foil of the same thickness--would crumple within a confined space, like a tube, before it would cease compressing under a certain weight. If I remember properly, it turned out the actual material being crumpled didn't matter, the effect was a geometrical one. μηδείς (talk) 16:25, 24 February 2013 (UTC)

The place to start on the engineering properties of crumpling would be to look up the physics of crumple zones that are the structural safety feature of modern cars. The point is that the crumpling action itself absorbs energy -- if the structure it is already partly crumpled, much of that absorbing potential is lost. So thin supports such as those inside cardboard are in an ordered honeycomb structure instead.

An interesting place to look at the fundamental physics of crumbling is in newer research on mathematical origami. An artist makes the incredibly-complex fold pattern on a computer, prints the pattern and burns the perforations on paper. But instead of assembling fold-by-fold, the artist finds that they can get most of the folding in place immediately by gently crumpling the sheet. SamuelRiv (talk) 16:39, 24 February 2013 (UTC)

Yes, both crumple zone and mathematical origami are interesting articles, and I have watched a documentary on the latter. But I am looking specifically for some sort of fractal treatment of the random crumpling of paper and other two-dimensional substances. μηδείς (talk) 21:33, 24 February 2013 (UTC)

Quantum mechanical stability of historical timelines

"The Moving Finger writes: and, having writ,

Moves on: nor all thy Piety nor Wit

Shall lure it back to cancel half a Line,

Nor all thy Tears wash out a Word of it."

- Rubaiyat of Omar Khayyam

How well does that stand up against quantum mechanics?

Is the past in any way fixed or is it instead a swirling storm of uncertainties that we don't notice simply because we're just the product of the current history and so can't see the pasts that used to be? Hcobb (talk) 03:53, 24 February 2013 (UTC)

It's impossible to tell. Whoop whoop pull up ^{Bitching Betty | Averted crashes} 04:28, 24 February 2013 (UTC)

Further reading I found while looking for this: Transactional interpretation. Hcobb (talk) 04:44, 24 February 2013 (UTC)

I've got two tests for the thread of history being in a vibrational state.

• Dark Matter might be echoes of past states of the past and so its fine scale structure could have certain harmonics.
• The Big Bang if connected to the present could have its structure influenced by the current configuration of the universe.

Hcobb (talk) 04:55, 24 February 2013 (UTC)

• You may want to check your ideas against this standard. --Jayron32 05:49, 24 February 2013 (UTC)

I think this is a deep and important question. We live in an indeterminate universe, and just as one present might be mapped to many futures, it might be mapped to many pasts. It seems like the religion of causality seems to suppose that a Watchmaker God decreed the past in some arbitrary and uninformative way, and the present is random, and the past is set in stone; but it would seem to me that the past is indeterminate, and the present is what we know. Indeed, I suspect you could argue solipsistically that whatever consciousness is - if it relies on some algorithm to somehow provide it with input - that it is impossible to distinguish the nature of the universe beyond what is consistent with the input of the present moment, and all the rest of the physical certainty, even memory, is an illusion, no? Wnt (talk) 07:58, 24 February 2013 (UTC)

There is no backward causality in quantum mechanics. -- BenRG (talk) 22:44, 24 February 2013 (UTC)

Would german V-2 rocket left earth orbit?

If a V-2 rocket would have been launced straight vertically with a 100 kg payload. Would it stay in space or fallen down ?, in particular would it left earth gravity for planetary space? Max altitude is specified as 206 km. Electron9 (talk) 05:50, 24 February 2013 (UTC)

This sounds like homework, but here's a hint: find out what the maximum velocity would be with a 100 kg payload and compare that with the escape velocity at your launch latitude. Zoonoses (talk) 06:59, 24 February 2013 (UTC)

Data on height vs velocity and escape velocity at specific height is missing.. Electron9 (talk) 07:53, 24 February 2013 (UTC)

You might like to read our Low Earth orbit article. Alansplodge (talk) 09:23, 24 February 2013 (UTC)

The article you link states that 206km is the maximum altitude if launched vertically. If it were capable of leaving the Earth's orbit, there would not be a maximum altitude, so it must fall down. Someguy1221 (talk) 09:26, 24 February 2013 (UTC)

So no secret Nazi base on the Moon? Clarityfiend (talk) 11:23, 24 February 2013 (UTC)

The first rocket reaching a low orbit with such payload was an R-7 Semyorka (280 tons) with its payload Sputnik 1 (83.6 Kg) in 1957. A V-2 was only 12,5 tons. --Kharon2 (talk) 21:31, 24 February 2013 (UTC)

Some book I read in high school many years ago said that the Germans in WW2 could theoretically achieved orbit with a V2 as the 1st stage, a cluster of some type of antiaircraft rockets as the 2nd stage and one such antiaircraft rocket as the third stage, implying that one stage to orbit was impossible for the V2. Getting the timing and steering to work is a different problem than thrust or specific impulse. Edison (talk) 22:15, 24 February 2013 (UTC)

A V-2 rocket couldn't possibly reach orbit -- but the proposed V-10 "Amerika-Rakete" two-stage version just might have. 24.23.196.85 (talk) 00:33, 25 February 2013 (UTC)

Something to remember is that V-2 is designed for a 1000 kg payload. If this is reduced, other orbits should be possible. Electron9 (talk) 02:37, 25 February 2013 (UTC)

Liquid nitrogen and frostbite

The image on the right shows liquid nitrogen is boiling and a person put his hand in the gas. Since the boiling point of liquid nitrogen is −196 °C, I am wondering why the person is not getting frostbite in his hand? The temperature of the newly formed gas is supposed to be −196 °C. --PlanetEditor (talk) 06:44, 24 February 2013 (UTC)

They might, eventually. However, being a gas, it has a low density and doesn't have much heat capacity. If he submerged his hands in the liquid, he'd get frostbite immediately. Try putting you hands in the refrigerator at 40 degrees F, then submerging them in water stored in the same refrigerator (which has reached 40°F), and you will notice how much quicker your hands get cold in the liquid. That said, having bare hands that close to liquid nitrogen is foolhardy. He should wear thick leather gloves. One slip and he will be seriously injured. StuRat (talk) 07:46, 24 February 2013 (UTC)

(ec) There are two important things about liquid nitrogen that make this less likely. First, low heat capacity - it doesn't store as much "cold" as it would seem - I'm getting "1.341j/g/K (gas), 2.042j/g/K (liquid), enthalpy of vaporization: 198.8j/g" from [16]. Our article properties of water says water has heat capacity 4.2 J/g/K around the freezing point, so you heat up the boiled nitrogen gas 3.1x faster than water, plus it absorbs the equivalent of 152 degrees when boiling, so it's sort of like "3.5 times colder than ice water" in terms of the energy it absorbs from your hand, in terms of hypothetical heat absorption.

But second, the liquid nitrogen surrounds itself in gas which rapidly pushes away from the skin, making it very hard to bring that full cooling to bear on it; there's poor heat conduction in a gas. It's a bit like a firewalk in that regard. I've held little drops of liquid nitrogen in my hand, but as the skin cools, they start to bite a little. If some prankster loaded a container of ethanol into the vessel holding the liquid nitrogen so that the demonstrator dipped his hands into that, I think there would be immediate injury. Wnt (talk) 07:52, 24 February 2013 (UTC)

The ethanol would freeze, of course. It melts at 159 K, whereas nitrogen boils at 77 K. Maybe you mean it would get cold enough to cause immediate injury before it froze? I suppose that's possible. But the violently boiling nitrogen around the container should be a clue. --Trovatore (talk) 08:00, 24 February 2013 (UTC)

You have a good point there. I suppose I was thinking that ethanol that has been sitting on dry ice is already quite cold enough! Wnt (talk) 08:44, 24 February 2013 (UTC)

Well their hand isn't really in the liquid nitrogen, probably just in the boiled off nitrogen and water vapour from the air. Making icecream with LN2 is very safe providing you know what you are doing. Your hand can get a bit cold when making ice cream like this over the minutes but unlikely to be so cold so as to cause any damage. When dipping your bare hand into liquid nitrogen you are protected by the Leidenfrost effect for several seconds so it feels cold but little heat transfer takes place. Damage is much more likely if the nitrogen gets stuck next to your skin for a longer period such as if it gets accidently poured in your glove or shoe and these can't be removed quickly. JMiall₰ 10:23, 24 February 2013 (UTC)

Thanks everyone. --PlanetEditor (talk) 13:26, 24 February 2013 (UTC)

One more interesting and important concept: the Leidenfrost effect. It's how people can stick their hand into boiling lead, for example. SamuelRiv (talk) 16:20, 24 February 2013 (UTC)

T-90 tank shtora system

Can Shtora system disable javelin missile ? 87.236.232.97 (talk) 08:19, 24 February 2013 (UTC)

It seems not, because Shtora "disrupts semiautomatic command to line of sight (SACLOS) antitank guided missiles, Laser rangefinders and target designators." The FGM-148 Javelin uses "an imaging infrared seeker". According to the China Defense Blog, "the Shtora is less effective against the more advanced third generation of ATGM such as the US Javelin or the German PARS 3 which relies on laser or electro-optical imager for guidance". Alansplodge (talk) 09:20, 24 February 2013 (UTC)

String or Higgs boson

Which fundamental particle give mass to all other particle? Is it string or Higgs boson? 106.215.104.44 (talk) 10:04, 24 February 2013 (UTC)

See higgs boson, higgs mechanism. "String" is not a particle, but a hypothetical model for all known particles. See string (physics). The higgs boson could hypothetically be a string. Someguy1221 (talk) 10:15, 24 February 2013 (UTC)

I guess you mean gives mass to other fundamental particles rather than all other particles i.e. not composite particles like a proton for the most part (see Proton#Quarks_and_the_mass_of_the_proton). Sean.hoyland - talk 10:33, 24 February 2013 (UTC)

Name of all 11 dimensions of space

I read in M-theory article that there are 11 dimensions of space. What is the name of all 11 dimensions? One is time and other 10 are ........ I read further in an another article that elementary particles are zero-dimensional. Then, what is the number of dimensions of an atom? 106.209.220.49 (talk) 11:58, 24 February 2013 (UTC)

To answer the first part of the question, space is four dimensional with the three spatial dimensions we are familiar with — you can call them length, width, and height or X, Y and Z — and time being the fourth dimension — see: Dimension (mathematics and physics)#Additional dimensions. It is not known whether any others exist, different theories posit 10, 11 and 26, so they haven't been named yet. Richerman (talk) 15:07, 24 February 2013 (UTC)

Also, it's useless to try to assign them names when we don't still know what they "look like". For example, they could be circular, so that walking along the "Q-axis" (a dummy name: t,x,y,z,q,l, etc) will quickly bring you right back where you started. For anything larger than the tiniest particles (e.g. gravitons), this "Q" is virtually non-existent. Or it could be like a sphere with "L" and "M" corresponding to latitude and longitude, again only fat enough for tiny particles to experience. One thing an extra dimension almost certainly is not, however, is like normal spatial length-width-height. If it were, we'd have seen it by now. SamuelRiv (talk) 16:18, 24 February 2013 (UTC)

This "names" thing is fairly silly in my view.

For spacetime (@Richerman: only spacetime is four-dimensional; space is still three-dimensional), you have one distinguished dimension which may be called "time". More precisely, there are "timelike" and "spacelike" intervals, and all observers will agree on which is which.

The remaining three, however, have no fixed "names". The names all correspond to a choice of (possibly local) coordinate system. For example, on the surface of the Earth, you'll probably use up/down for one axis, but whether you use left/right forward/backwards, or port/starboard fore/aft, or north/south east/west, depends entirely on your immediate purpose.

For what it's worth, and it isn't worth much, a writer by the name of Charles Howard Hinton proposed the names ana and kata for opposing directions in a fourth spatial dimension. How, or even whether, he proposed to distinguish between them, I am unaware. --Trovatore (talk) 00:05, 25 February 2013 (UTC)

Food energy

The standard food energy values, as given in our article and appears in most publications, are as follows:

• carbohydrate: 4 kcal/g
• protein: 4 kcal/g
• fat: 9 kcal/g

On the other hand, as given here and mentioned this journal article, energy value of fat is 9.3, and that of protein is 4.2 for meat protein and 4.3 for vegetable protein.

Now, this article published by the USDA says (p.4) energy value of protein is 5-6 kcal/g. This also describes the energy value of protein is 5.6 kcal/g. Why so many variations? I cannot understand the reason for the differences. --PlanetEditor (talk) 13:13, 24 February 2013 (UTC)

• I found this book which says "in vivo, the oxidation of protein is only party complete .... calorific value of protein, in vivo, is less, i.e. 4.1 Kcal/gm". The the book does not cite any reference. Definitely the USDA article is the most reliable. --PlanetEditor (talk) 13:20, 24 February 2013 (UTC)

• The main point here, is there any way to know whether the energy value of a food, as determined by bomb calorimeter, is different from energy value of the same food in human body? --PlanetEditor (talk) 13:36, 24 February 2013 (UTC)

It obviously is not. A bomb calorimeter can burn sawdust. A human body can not. The ash that a bomb calorimeter leaves behind cannot be ignited and burned. Dried dung can. The question is what the calorimeter operators do to compensate for these sources of error. See [ http://www.fishersci.com/ecomm/servlet/cmstatic?href=Scientific/researchAnalytical/ProductsServices/Food_Diagnostics/food_beverage_newsletter_bombcal.jsp&store=Scientific&storeId=10652 ]. --Guy Macon (talk) 19:49, 24 February 2013 (UTC)

Very interesting question; my first instinct would be that the caloric values given are for proteins/carbs/etc. which have already been digested and are being oxidized in the mitochondria, but I can't find a reference for that; note that the efficiency of digestion, for lack of a better phrase, varies between species and foodstuffs; cats don't extract as much from their food as goats, for instance. Gzuckier (talk) 06:26, 25 February 2013 (UTC)

Our mirror image is chiral or achiral

What is the nature (chiral or achiral) of our image formed in a mirror? 106.209.220.49 (talk) 13:25, 24 February 2013 (UTC)

This question indicates that you don't understand the meaning of the words "chiral" and "achiral". A chiral object is an object whose mirror image can't be superimposed on the original. An achiral object, in contrast, has a mirror image that matches the original. Looie496 (talk) 16:13, 24 February 2013 (UTC)

Then, what about our image formed on mirror? I want to know whether this image is able superimpose our body (achiral) or it isn't (chiral). 106.209.220.49 (talk) 16:56, 24 February 2013 (UTC)

If you are chiral, then your mirror-image is chiral. If you are achiral, then your mirror-image is achiral. --catslash (talk) 17:34, 24 February 2013 (UTC)

You may be interested in Julian Wolkenstein's Symmetrical Portraits series and his echoism.org site. Sean.hoyland - talk 17:46, 24 February 2013 (UTC)

If you part your hair on the right, the person in the mirror parts his hair on his left. If you take a picture of yourself and take a picture of yourself in the mirror, when you try to superimpose the pictures on each other the parts of the hair will be on opposite sides. So you are chiral. Even if you part your hair in the middle, you are still asymmetric and therefore chiral. Duoduoduo (talk) 18:14, 24 February 2013 (UTC)

Vegetarian Experience

Last summer I decided to be a vegetarian, and since then, something weird happened to me, I began to have dreams of me eating meat, and after a while I began to feel guilty about it in the dream, and in the last dream I had I actually spitted out the meat. I was just curious is it common/normal? Is it a way for my body to tell me I need meat? (Probably like sexual dreams?) I'm interested to know what's happening in my brain... Thanks!--Irrational number (talk) 15:15, 24 February 2013 (UTC)

I have taken the liberty of removing the first two copies of this question. It looks like something got screwed up with the post. Falconus^{p t c} 15:17, 24 February 2013 (UTC)

Thank you, I clicked on the "save page" button more than once, very slow connection here this is embarrassing... I feel like a cave man... — Preceding unsigned comment added by Irrational number (talk • contribs) 15:22, 24 February 2013 (UTC)

This falls into the category of "wish fulfillment" dreams -- a major type. (Freud thought that all dreams are of that type.) It simply indicates that you had a strong desire to eat meat. Looie496 (talk) 16:15, 24 February 2013 (UTC)

To continue using Freud, the desire to eat the meat comes from your id, while the desire to avoid it comes from your superego (which aims for perfection), and these are mediated by the ego. This conflict is being fought out in your dreams. StuRat (talk) 16:48, 24 February 2013 (UTC)

I think it might have been more normal if you had spat out the meat, rather than sticking it on spits. Food cravings are usually conscious. If your dreams are bothering you you need to see a medical professional. We can't do oneiromancy here. μηδείς (talk) 16:17, 24 February 2013 (UTC)

There's nothing in the question that would indicate that this is a wish fulfillment dream, or that Irrational number has a strong desire to eat meat. The only emotional reaction to eating meat that was expressed in the question is guilt, and the dream behavior of spitting out the meat would be consistent with disgust. Red Act (talk) 18:12, 24 February 2013 (UTC)

I believe it's normal. My own experience tells me that - I once gave up meat for Lent, and by Good Friday I was getting phantom roast chicken smells. Also my vegetarian friends tell me they still get cravings for meat, most commonly bacon. As to why this happens, I don't think there's any research into it that will tell you. So don't worry about it. --TammyMoet (talk) 16:37, 24 February 2013 (UTC)

There's no mystery about the attractiveness of the smell of roast chicken or bacon. See Maillard reaction. Bacon is a superstimulus, it's basically the essence of meat all in one high-fat crunchy protein package:again, Maillard reaction as well as Monosodium glutamate, etc. μηδείς (talk) 18:23, 24 February 2013 (UTC)

I would take the desire to eat meat as a sign that you're not getting enough protein (or possibly some other nutrient, like iron or Vitamin B12). If you want to remain a vegetarian, I suggest beans and nuts. (You might want to get some Beano, if you plan on getting your protein from beans without offending everybody you know.) If your version of vegetarianism permits eggs and/or dairy products, those are other options. My (extremely lax) version also permits fish. If iron or B₁₂ is the deficiency, there are other nutritional sources of those, too. StuRat (talk) 16:58, 24 February 2013 (UTC)

Re: "We can't do oneiromancy here", sure we can. We aren't allowed to give medical or legal advice, but oneiromancy, like other forms of divination, is certainly allowed, despite the control issues some folks here have concerning what other people post. And who better to interpret your dreams than some random stranger on the Internet? I agree with StuRat that cravings for meat and/or dreaming about meat may very well be a sign of an inadequate diet. It is a well-known fact that, while a vegetarian or even vegan diet can be healthy and balanced, you have to work at it. Wikipedia has some excellent info on this issue at Vegetarian nutrition#Potential nutrient deficiencies and Vegan nutrition#Nutrition. --Guy Macon (talk) 18:01, 24 February 2013 (UTC)

However, if the OP starts growing towards the light, he should see a doctor. Or maybe a gardener. ←Baseball Bugs ^{What's up, Doc?} carrots→ 20:17, 24 February 2013 (UTC)

I quit smoking at the start of the year and I've had a few dreams of me just chain smoking. Vespine (talk) 21:46, 24 February 2013 (UTC)

Actually, the OP is asking for medical advice, which is not allowed. However, any kind of change can lead to what I can "anxiety dreams". But if you're really concerned that there might be something wrong with you, see your doctor. ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:17, 25 February 2013 (UTC)

Correct me if I'm wrong but I think the definition of medical advice in the reference desk is getting wider. I didn't ask what I should do, I was just curious about how common this is and what is happening in my brain and why. Is any question about human body a request for medical advice? I think a request for advice must at least have a part in which the OP asks what they should do, which I didn't.--Irrational number (talk) 10:40, 25 February 2013 (UTC)

stereoscopic images

How much did it cost to make them in the 1800s' — Preceding unsigned comment added by 172.218.36.253 (talk) 20:45, 24 February 2013 (UTC)

About twice as much as it took to make a normal photograph. Back then the one-time equipment costs were much higher (new technology) and the per-image costs comparatively lower (they re-used plates instead of buying more film). --Guy Macon (talk) 21:42, 24 February 2013 (UTC)

Refs for stripping the emulsion off plates and re-using them? I thought plates were mostly one-time use. Edison (talk) 03:54, 25 February 2013 (UTC)

Rate of exothermic reaction

Let's assume one wishes to measure the rate of reaction for an exothermic reaction in aqueous solution. Would measuring temperature of the reaction and graphing the temperature at given intervals be an accurate way to measure this? Is there a given point where the reaction would be able to be determined "finished"? And if this is a viable method, what type of container would be most useful? Must one use a calorimeter, or just any beaker or flask? 66.41.216.19 (talk) 22:36, 24 February 2013 (UTC)

Well, the temperature will be cumulative, ignoring cooling to the environment, so you might want to graph the change in temp versus time, rather than the temperature itself. An insulated container would be best, to minimize the effect of cooling to the environment. However, beware that some reactions might get out of hand, leading to an explosion, in such a situation, as the insulated container may increase the temperature, and thus the reaction rate. The reaction is finished when the temperature change is zero (smaller time increments provide better data). StuRat (talk) 22:57, 24 February 2013 (UTC)

So you're saying I should record the change in temperature over each, say, 5 second period until the temperature ceases to change? I was only planning on reacting zinc with 3.0 M HCl, so an explosion risk is low. However, I am not sure if a calorimeter would affect the rate of reaction as you just said. If so, how so? Would I have to know the calorimeter constant? Or would I be better off with a beaker? I'm just trying to find a way one could measure rate of reaction between these particular compounds. 66.41.216.19 (talk) 23:06, 24 February 2013 (UTC)

Well, most reactions are sped up by higher temps, but there may be a few exceptions (say if the higher temperatures cause one of the reactants to change into a gas, and leave the reaction vessel). Now, why exactly do you want to know the speed of the reaction ? If there was a Q you were trying to answer, like "does the color change take place during the reaction or after it has completed", then speeding it up wouldn't matter. If, on the other hand, your question is "how long does the reaction take to complete at STP", then you not only need to avoid an insulated container, but you might want to cool it to standard temperature during the reaction, depending on how you interpret the Q.

Also note that in any sealed reaction vessel, you may also need to account for the effect of a change in pressure, as that can change the reaction rate, too.

Is every 5 seconds the fastest you can measure temps ? If you are doing it manually, that's about right, but automated temperature probes/recorders ought to do far better than that. StuRat (talk) 23:38, 24 February 2013 (UTC)

Maybe once per second with the technology I have available. I want to find out the rate of the reaction to investigate the effect of surface area of Zn(s) on rate. I would use solid lumps of zinc, zinc filings, and zinc powder in a constant amount (molarity) of HCl. Rate is my dependent variable, and form of zinc is my independent variable. The rate of reaction, I thought, might be the initial number of moles of Zinc (constant for each trial set) divided by the amount of time before the temperature ceases to change. Is there an easier way? 66.41.216.19 (talk) 23:46, 24 February 2013 (UTC)

About how long does this reaction take, in the slowest case ? The only problem with the open reaction vessel is that, when the reaction is trailing off, the small amount of heat produced may equal that lost to the environment, so it will appear that the reaction ended earlier than it did. You might want to try it both in the bomb calorimeter and in the open vessel, to see if the ratio is different between the fastest and slowest reactions. If not, just stick with the open vessel. StuRat (talk) 23:52, 24 February 2013 (UTC)

I have a styrofoam calorimeter with a small hole in the lid to permit gases to escape, therefore keeping temperature loss at a minimum while keeping pressure constant. Is this OK? (thnx for ur patience) 66.41.216.19 (talk) 23:54, 24 February 2013 (UTC)

Sounds reasonable to me (provided that nothing will react with the Styrofoam). Of course, keep in mind that any experiment is only a model of "the real world case". So, for example, if you want to know which is faster for designing an industrial reaction process, then you'd want to set up your experiment as closely to the industrial process as possible. StuRat (talk) 00:03, 25 February 2013 (UTC)

Better quality safety glasses

I noticed when I buy clear safety glasses they don't have very good clarity to them and seem to stop light transmission to some degree. They also get a lot of glare on them. I don't wear prescription glasses but I have friends that do I have looked through their glasses and the plastic they are made from has very good light transmission and is extremely clear. They also seem to get almost no glare. I'm wondering if you can buy safety glasses with the same quality lenses that they make prescription glasses from. --Tommythehook (talk) 23:47, 24 February 2013 (UTC)

Well, plastic will get tiny scratches over time, which will make them look foggy. Glass won't do that. Of course, glass could be quite dangerous, if it shatters. So, there are special types of glass which are less likely to shatter, which would be suitable for safety glasses. StuRat (talk) 23:57, 24 February 2013 (UTC)

Most prescription glasses are not made from glass. Also my safety glasses have problems brand new, its not due to scratches.--Tommythehook (talk) 00:11, 25 February 2013 (UTC)

There are 2 reasons why presciption specs are better: 1) anti-reflection coatings, and 2) frames for prescription use have the lenses closer to the eye - thus making surface imperfections further out of focus and so less noticeable. This question has come up before, for a possible solution - see http://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Archives/Science/2012_June_15 Ratbone 124.178.54.230 (talk) 01:12, 25 February 2013 (UTC)

I'm pretty sure it's the same banned sockpuppet so I guess the previous answers didn't help. Nil Einne (talk) 05:11, 25 February 2013 (UTC)

February 25

Pot stats in USA

Hello all, I'm trying to find a comparative breakdown of the marijuana abuse rate trends state-by-state from 1998 to present. Here are the requirements I have for this data: (1) it must be from a reputable source; (2) it must show the marijuana abuse rates separately for each of the 50 states; (3) it must show these rates over several years, preferably over the entire period from 1998 to present; and (optional but preferred) it wouldn't hurt if it indicates which of the states have legalized pot, and in what year. I've been trying to find it for days, but all the studies I've found either aggregate the data over the entire country, or else they only have the data for one year (two at the very most). Thanks in advance! 24.23.196.85 (talk) 00:44, 25 February 2013 (UTC)

Not that I have an answer in any case, but I think you need to specify what you mean by "abuse". Do you mean, say, (1) any recreational use, (2) recreational use in violation of state law, (3) usage that causes medical harm or makes it difficult for the user to discharge his ordinary responsibilities? The most justifiable sense of the word "abuse" is number 3, but it's also a bit vaguely defined, so precise stats would depend on who is judging. --Trovatore (talk) 00:58, 25 February 2013 (UTC)

Because marijuana is largely illegal, it's almost impossible for precise figures on anything to do with its use to be collected. HiLo48 (talk) 01:15, 25 February 2013 (UTC)

Yes, that's true. Still, there is an extra layer of ambiguity added by the term abuse. On the other hand, I suppose it's possible that there are avenues to track genuine abuse (sense 3 in my list) that are not available for private use that doesn't cause obvious harm. For example, you could interview doctors and ask them to provide aggregate statistics, without identifying any individual. --Trovatore (talk) 01:20, 25 February 2013 (UTC)

I actually meant "abuse" in sense (1). 24.23.196.85 (talk) 01:28, 25 February 2013 (UTC)

Since that's not the normal use of the term "abuse", I suggest you use the term "recreational use", instead, when doing searches. StuRat (talk) 01:31, 25 February 2013 (UTC)

Anti-drug warriors are probably the only ones who use "abuse" in sense (1). ←Baseball Bugs ^{What's up, Doc?} carrots→ 06:14, 25 February 2013 (UTC)

or sense 4, insulting and belittling the substance until it feels bad about itself, or sense 5, physically injury to the substance Gzuckier (talk) 06:32, 25 February 2013 (UTC)

I remember a girl who would look for any opportunity to use the phrase "that's alcohol abuse!". Examples might be spilling beer or dropping a bottle of champagne. --Trovatore (talk) 08:19, 25 February 2013 (UTC)

To Mr. Bugs: Since when is there anything inherently wrong with being an anti-drug warrior? 24.23.196.85 (talk) 06:52, 25 February 2013 (UTC)

Ever since it became blatantly obvious to all rational people that the war on drugs was never going to reduce drug usage. HiLo48 (talk) 07:19, 25 February 2013 (UTC)

Well, that's not really all that logically connected — you could be a crusader in a hopeless cause and still be right. But the more pertinent question is, why should we consider all recreational use of marijuana to be "abuse"? Is there any decent evidence that the people who so consider it are right? --Trovatore (talk) 08:18, 25 February 2013 (UTC)

Nothing wrong with it, but I wouldn't look for unbiased statistics from anyone with a particular political agenda. I also wouldn't trust any figures from the participants in the Hash Bash. If you want reliable stats, look for them from uninvolved scientists, who won't tend to use subjective terms like "abuse". StuRat (talk) 07:20, 25 February 2013 (UTC)

Lithium ion batteries

Can electronics with lithium ion batteries be damaged if you don't use it enough or don't charge it fully before taking it off the charger? Clover345 (talk) 01:58, 25 February 2013 (UTC)

No, not if the battery is working correctly, but you can potentially shorten the life of the battery. Looie496 (talk) 02:57, 25 February 2013 (UTC)

How often does it need to be used or charged fully to prevent this? Clover345 (talk) 10:32, 25 February 2013 (UTC)

Does exercise really result in weight loss?

Science writer Gary Taubes says exercise has no effect on weight loss. On the other hand, the mainstream medical establishment believe in "calorie in-calorie out" theory of adiposity, i.e. if you spend more energy than you eat, you will be lean, if you eat more than you spend, you will be fat. Is it really true exercise does not result in weight loss? If so, then why do we see so many people who claim they lost weight after exercising? I could not find any scientific criticism of Taubes' claim regarding exercise. Is his claim right or wrong? If Taubes' theory is right, then how could the heaviest person Manuel Uribe lose 400 kg (597 kg to 200 kg) after diet and exercise program? And if Taubes is wrong, why there is no scientific criticism? --PlanetEditor (talk) 03:42, 25 February 2013 (UTC)

This is very individual. I personally have definitely had periods of long-term weight loss success based mainly on exercise. But it all depends how your calorie intake responds to an exercise program. Some people may get hungrier and eat more; other people may find that the exercise relieves stress that they had been treating with food, so they actually eat less (or at least not much more). --Trovatore (talk) 03:46, 25 February 2013 (UTC)

It very much depends on your assumptions:

1) If you start to exercise, without changing anything else, specifically your diet, then you should lose weight (or at least gain it more slowly than before), as the same number of calories are going in, and some of those are now being burnt by exercise.

2) If you also increase the calories you consume, because the exercise makes you hungrier, then you might still lose weight, see no change, or even gain weight, depending on if the additional calories consumed are more or less than those burned by the exercise.

However, in any case, exercise should make you healthier, by converting fat to muscle, etc. (assuming you don't pick a dangerous or injury-prone exercise). Also note that the fastest way to lose weight is to both cut calories and exercise, although such rapid weight loss may not be healthy. StuRat (talk) 03:49, 25 February 2013 (UTC)

This is a common question on Ref Desk, and StuRat has given you a good common sense answer. For lots of discussion on this, search Ref Desk archives for "exercise weight loss" (see the search filed at the top of this page). See for example http://e.wikipedia.org/wiki/Wikipedia:/Reference_desk/Archives/Science/2010_July_1#Weight_loss. I'll summarise here: If, over a sustained period of time, you eat the same calorific value as you did before, but increase your physical effort, then the increased mechanical energy you expend has to come form somewhere - if not from the food, then it MUST come from metabolising body mass - there is no other source.

I worked for a few years for a large company who had a large office tower. Floors 1 and 2 were a calls centre - about 200 women sitting at desks answering phones. Floor 3 was the staff canteen. Call centres are renowned for fat workers, due to sitting on their arse for hours at a time doing nothing but talk and click the mouse, maybe push the odd keyboard key. But some women were trim and remained trim. Some women were fat and just got fatter the longer they worked there. Guess which ones walked up the stairs to go to the canteen to eat their subsidised food? Guess which ones always took the lift, just to go up or down one or 2 floors?. Guess which ones parked their cars in free parking a kilometer away and walked the remaining distance? Which ones used paid parking next door? Yup - you got it - its the fat ones who didn't do any walking - every time.

Wickwack 124.178.143.40 (talk) 04:38, 25 February 2013 (UTC)

Since nobody seems to have mentioned it, note that replacement of fat with muscle might not result in "weight loss", in that muscle is heavier than fat. Probably less important for a 400 pounder than for somebody trying to get from 175 to 170. Gzuckier (talk) 06:35, 25 February 2013 (UTC)

That will occur if the degree of exercise and food intake are sufficient. However, it does not, and can not, change what StuRat and I said above: If food intake is not increased, then over a sustained period of time, increased exercise MUST reduce your weight. In fact, weight will go down whenever a regime of food digestable calorific value greater than body metaboloic need + mechanical energy produced is sustained. The ratio of muscle mass to fat mass might well change, but total body mass MUST go down. In theory, there are a couple of factors StuRat and I did not cover (they were covered by people posting in response when this question was asked before - did you check the archives?): 1) Expelled faeces contain unburned calorific value; and 2) the metabolic uptake of the brain is dependent of how hard it works (including how hard you think/concentrate on problems). However, variation in brain metabolic uptake is not in practice great enough to make a noticeable impact on total body mass, unless you do silly thinks like use drugs to sleep most of the 24 hours in a day. The body also has minimal ability to improve digestion efficiency - and the reason why faeces have calorific value is becasue that calorific value is in things like celulose fibre that humans cannot digest. Wickwack 60.230.230.154 (talk) 07:06, 25 February 2013 (UTC)

And another minor factor is how well hydrated the person is. If they sweat heavily as a result of the exercise, and don't drink more fluids to replace those fluids, they may also lose weight by being dehydrated. Not healthy, of course. StuRat (talk) 07:14, 25 February 2013 (UTC)

True. And taking a duretic (herbal or prescription dug) will reduce weight. Eating more salt will, by causing water retention, increase weight. although in healthy people gnerally middle aged or younger, the kidneys are very effective in getting rid of excess salt. However, significantly modulating your weight by any of these methods will eventually cause other medical issues - as you said - not healthy. Wickwack 60.230.230.154 (talk) 07:24, 25 February 2013 (UTC)