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This is an old revision of this page, as edited by Bonkeyballs (talk | contribs) at 00:32, 15 December 2020 (Why do so many explosives feature the elements of life i.e. H, C, N, O?). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

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December 8

Covid vaccine

As I understand it, the basic reproduction rate (R0) of Sars-Cov-2 is around 5.0. Now there's a finding that the Astra-Zenica vaccine is about 70% effective.[1] Does that mean if everyone is vaccinated, 70% of the transmission is eliminated, so R0 drops to 0.3*5.0 = 1.5? And is that not still a contagious virus, especially if everyone drops the minimal precautions that they're taking now? I feel like we need a stupidity vaccine to go along with the Covid vaccine. Thanks. 2602:24A:DE47:BB20:50DE:F402:42A6:A17D (talk) 21:49, 8 December 2020 (UTC)[reply]

We don't know how any of the vaccines affect transmission. All that has been measured is protection against symptomatic infection. Graham Beards (talk) 22:01, 8 December 2020 (UTC)[reply]
They don't test for presence of the virus, as opposed to looking for symptoms? Lots of people test positive who are asymptomatic. They are still capable of transmission, from what I understand. Then there will also be the issue of multiple strains of the virus etc. Anyway, thanks. 2602:24A:DE47:BB20:50DE:F402:42A6:A17D (talk) 22:08, 8 December 2020 (UTC)[reply]
A problem with quoting specific values for the basic reproduction number (not "rate") is that it depends on the behaviour of individuals in the population; in a society where the typical indivual meets many others, shaking hands if not kissing, the number will be higher than in a less gregarious society whose cultural norms in greeting are more constrained. But in this context we should not look at the basic reproduction number, but at the effective reproduction number (before vaccination). Let's do some maths. I'll work with fractions instead of percentages. If the effectiveness of a vaccine is given as e, I take it to mean that, whereas a fraction s in the control group developed symptoms during a given period, that fraction was reduced by a factor 1−e to (1−e)s in the experimental group – those that were vaccinated. Among this group of asymptomatic vaccinees, some will nevertheless be infectious carriers. For simplicity I define "carrier" to mean "infectious carrier" – I think it will be rare for a carrier to be both symptomatic and not infectious, and we'll let asymptomatic non-infectious carriers fly under the radar. It is not unreasonable to assume that the ratio between symptomatic and asymptomatic carriers among vaccinees is similar to that in the general unvaccinated population. Denote the fraction of asymptomatic carriers among all carriers by a, so the number of asymptomatic carriers equals the number of symptomatic carriers time a factor a/(1−a). Among the experimental group, a fraction (1−e)s became symptomatic, which would imply a fraction (a/(1−a))×(1−e)s of infectious but asymptomatic vaccinees. To make any progress we need further assumptions: all individuals who develop symptoms self-quarantine, and the probability of a vaccinee becoming infectious and transmitting the disease before immunity would normally kick in is small enough to allow it to be neglected. In the same period, a fraction (a/(1−a))×s of infectious but asymptomatic vaccinees from the unvaccinated population will walk around. If a fraction v of the population is vaccinated, they will then contribute a fraction v×(a/(1−a))×(1−e)s of infectious but asymptomatic to the total population, whereas the unvaccinated individuals contribute a fraction (1−v)×(a/(1−a))×s; together v×(a/(1−a))×(1−e)s + (1−v)×(a/(1−a))×s = (v(1−e)+(1−v))×(a/(1−a))×s, compared to (a/(1−a))×s without vaccination (equivalent to setting v = 0). The reduction in ambulant infectious individuals is then by a factor v(1−e)+(1−v). As this simple calculation shows, to see the effect of a large-scale vaccination programme we do not need to know a numerical estimate for a – which is estimated to be about 0.4.[2] If e = 0.7 and v = 0.75, that comes out as 0.475. The effective reproduction number can be expected to go down by the same factor, which may be enough to quell the epidemic.  --Lambiam 12:14, 9 December 2020 (UTC)[reply]

December 9

Lifejackets and buoyancy center

I remember that when I used a lifejacket, it was harder for me to swim because the center of buoyancy at the neck level constantly pushed me in a direction perpendicular to swimming direction, acting as an opposite force. My gut feeling is that the lifejacket should be more like a belt so that the center of buoyancy would be placed lower, allowing more flexibility for swimming. Is is actually a legit concern physics-wise? Thanks. 212.180.235.46 (talk) 18:58, 9 December 2020 (UTC)[reply]

The way I learned how to use a life jacket, (or more precisely, a "personal floatation device"), swimming isn't really part of the purpose.
The goal of the PFD is to keep the survivor at or above water level until rescue arrives.
In almost every single use-case, the survivor should be upright and perpendicular to the water, as illustrated in the US Coast Guard PFD information webpage. In fact, especially in cold water, the guidance is specifically not to swim: "...don't swim unless you can reach a nearby boat, fellow survivor, or floating object. Even good swimmers drown while swimming in cold water. Swimming lowers your body temperature."
...In most of the realistic but hypothetical survival situations on the water, if you could actually reach that nearby boat by swimming, you probably wouldn't have needed the PFD in the first place.
Nimur (talk) 19:06, 9 December 2020 (UTC)[reply]
Some people would panic and drown without the vest, they need it even for a short swim to shore in warm water. Sagittarian Milky Way (talk) 22:20, 9 December 2020 (UTC)[reply]
I don't think your center of buoyancy could be at a neck level. As the linked article defines it, the center of buoyancy is a centroid of the displaced water. So it is a geometric center of the part of body below the surface, whilst the neck should be at the surface level to keep the head above water.... --CiaPan (talk) 21:03, 9 December 2020 (UTC)[reply]
Most corpses are seen floating face down thanks to buoyancy at belt level, see [3]. 84.209.119.241 (talk) 21:52, 9 December 2020 (UTC)[reply]
I don't remember where, but I've definitely read somewhere that early lifejackets were dangerous if the wearer fell unconscious, precisely for this reason. Without a definite effort to keep the head above water, the wearer tended to turn face down. --174.95.161.129 (talk) 23:03, 9 December 2020 (UTC)[reply]
In UK terminology, a lifejacket keeps you afloat when you're unconscious, whereas a buoyancy aid helps you to float but allows you to swim. Therefore, in situations where you're likely to have to swim after falling in (i.e. kayaking or windsurfing), use a buoyancy aid; if you're likely to be in the water for a while (if you abandon a ship or aircraft for instance) use a lifejacket. Ditto if you can't swim. BTW, I'm a kayak coach. On some American films I've seen things that look like a combination of both, so it may be a bit different on the wrong side of the pond. See also Buoyancy Aids & Lifejackets Alansplodge (talk) 00:20, 10 December 2020 (UTC)[reply]
To be more precise: a lifejacket is designed to keep your face out of the water if unconscious. (Briefly mentioned here). Iapetus (talk) 10:57, 10 December 2020 (UTC)[reply]
Quite right, I didn't word my reply very well. Generally a lifejacket supports you in a head-back posture, which is rubbish for swimming except for a leisurely backstroke. Alansplodge (talk) 21:01, 10 December 2020 (UTC)[reply]
The stabilising jackets used by scuba divers are so bad as life jackets that some of them will actively force you into a face-down position at the surface. It's just about impossible to swim any other style than on-the-back, legs only in any of them and this is certainly what I'd try with any other form of bouyancy aid or life jacket. 2A01:E34:EF5E:4640:8848:C97D:6ACB:2E9C (talk) 16:15, 11 December 2020 (UTC)[reply]
The various styles of kayaking buoyancy aids that I have used all allow for a respectable breaststroke as well as backstoke. The requirement to be able to swim strongly in whitewater is rather important to avoid being smashed into rocks and other unpleasantness. Alansplodge (talk) 12:35, 12 December 2020 (UTC)[reply]
Inflatable armbands or "water wings" allow more flexibility for swimming, especially for children under supervision. 84.209.119.241 (talk) 16:24, 10 December 2020 (UTC)[reply]

Why does Richard Bowie Spikes and Jan Ernst Matzeliger have the same image?

Why does Richard Bowie Spikes and Jan Ernst Matzeliger have the same image — Preceding unsigned comment added by 2601:644:4080:9A70:E503:9DF2:FFC6:D638 (talk) 23:57, 9 December 2020 (UTC)[reply]

They don't. Jan Ernst Matzeliger has an image, but Richard Spikes does not. Also what does this have to do with the science reference desk? --OuroborosCobra (talk) 00:44, 10 December 2020 (UTC)[reply]
Are you by any chance referring to a photo or text shown to the right of a Google search? Google's Knowledge Graph uses a wide variety of sources. There may be a text paragraph ending with "Wikipedia" to indicate that particular text was copied from Wikipedia. An image and other text before or after the Wikipedia excerpt may be from sources completely unrelated to Wikipedia. We have no control over how Google presents our information, but Google's Knowledge Graph has a "Feedback" link where anyone can mark a field as wrong. The same feedback facility is also provided on Bing and some other search engines..--Shantavira|feed me 10:16, 10 December 2020 (UTC)[reply]
Yes I strongly suspect the OP is referring to the Google Knowledge Graph which does show the same image for both. The ultimate answer to this is that you'll have to ask Google's engineers why. However it's perhaps not particularly surprisingly since a number of sources seem to use the image we have for Jan Ernst Matzelinger or similar images for Richard Spikes e.g. [4] [5]. The image in our article for Jan Ernst Matzelinger seems to have originated from Encyclopaedia Britannica for our article. Hopefully they are right about the identification, it seems that image has been used for stamps and stuff [6]. Other images for the two are this for Richard Spikes [7] and this for Jan Earnst Matzelinger [8]. Nil Einne (talk) 12:04, 10 December 2020 (UTC)[reply]
Neither of the latter two photos looks to me like they could be of the same person as in the EB photo.  --Lambiam 23:03, 10 December 2020 (UTC)[reply]

December 12

Pressure cooker

I have an electric pressure cooker. Let's say I heat up several liters of water in it, so it is at 1 atm of steam pressure. So there is liquid and pressurized gas in the pot, at (I believe) a temperature of about 125 celsius. Now I release the steam and open the lid. There is liquid water in the pot, bubbling somewhat, which means it is at 100C. There was no cloud of superheated steam or anything like that.

Does that mean the water somehow cooled from 125C to 100C? Just from the gas expansion of the steam being let out? It seems like an awful lot of heat to shed that way. Is some other part of the picture wrong? If not, where did the heat go? Thanks. 173.228.123.178 (talk) 23:27, 12 December 2020 (UTC)[reply]

Added: I guess some of the liquid inside turns to vapor when the pressure released, letting out heat of vaporization which for water is pretty high, but I'm still not sure that explains everything. 173.228.123.178 (talk) 00:13, 13 December 2020 (UTC)[reply]

You answered yourself when you said "Now I release the steam". Inside the pressure cooker there will be a furious evolution of steam as the superheated water turns to steam and the heat of vaporisation is taken from the thermal energy of the remaining water, which cools down in consequence. From memory my mother let the pressure cooker cool before releasing the steam, the resulting food put me off pressure cookers for life. Greglocock (talk) 00:41, 13 December 2020 (UTC)[reply]

The water at 125C will boil furiously and probably foam up enormously if the pressure is suddenly released, this will remove heat from the water, but is very dangerous, so usually you cannot safely take the top off a pressure cooker. Also if you are high in elevation it will boil at less than 100°C. Graeme Bartlett (talk) 11:39, 13 December 2020 (UTC)[reply]
There are many PC recipes that call for releasing the steam early (so-called quick release). You don't take the top off, you press down on a button and that lets the steam shoot through a little valve. I think I understand now though: the heat is taken away by water vaporization and the steam exiting. If my math is right, about 5% of the water mass should be vaporized by this process. That shouldn't be too hard to check experimentally with a kitchen scale (just weigh the water before and after). I might try it sometime. 2602:24A:DE47:BB20:50DE:F402:42A6:A17D (talk) 09:15, 14 December 2020 (UTC)[reply]
Yes, many kitchen scales have a plastic or glass pans, which may melt or break if you put a thick, hot pot on them. --CiaPan (talk) 10:56, 14 December 2020 (UTC)[reply]
Good point. A wood chopping board or any 1" thick piece of wood would be a wise precaution Greglocock (talk) 21:36, 14 December 2020 (UTC)[reply]

What's the least soft thing you could cut with an atomically perfect diamond without breaking a bond?

2. If gem trade perfect diamonds (AKA 10x loupe) were big and cheap instead of about an inch wide at best and 8 digits of dollars could you make a practical all-diamond razor-sharp point and edge food knife blade with lasers? How much twisting or bending force could it take and could you use it as a heavy cleaver or nutcracker and stab pouring holes in a coconut with 1 stab per hole side with it or is the limit closer to cutting tofu? Sagittarian Milky Way (talk) 23:34, 12 December 2020 (UTC)[reply]

To answer what I think is your second question, diamond would make a terrible cleaver or mallet. As our article Material properties of diamond says, "Unlike hardness, which denotes only resistance to scratching, diamond's toughness or tenacity is only fair to good. Toughness relates to the ability to resist breakage from falls or impacts. Because of diamond's perfect and easy cleavage, it is vulnerable to breakage. A diamond will shatter if hit with an ordinary hammer." CodeTalker (talk) 19:26, 13 December 2020 (UTC)[reply]
Is it at least harder to break than a cheap glass blade? Sagittarian Milky Way (talk) 16:12, 14 December 2020 (UTC)[reply]
Diamond knife suggests that the last thing one needs to use is gem-quality diamonds! Mike Turnbull (talk) 12:35, 14 December 2020 (UTC)[reply]
Right, if you want a knife with diamond's hardness you just take a metal knife and encrust with crushed junk diamond. If you want to cut tofu with a novelty bling you go to a parallel Earth where top grade (D flawless) stones many times the world record width exist and cut a blade shape with lasers. Then you do as much of your kitchen work with it as you can without cracking it till the novelty wears off or maybe it might even never get old. Sagittarian Milky Way (talk) 16:09, 14 December 2020 (UTC)[reply]

December 13

Cannabis plant does itself adjust/move how the sunlight runs

Hello there. I have seen on a balcony YouTube video that the planted cannabis plants do move their selves to get as much light as possible and also "fall" themselves asleep after the sunset. I am interested which molecule / pheromon / nerves recipe does inside the plant is activated to make the plant align to the sunlight the same way sunflowers are able to turn in circle to align to the sunlight? Where do these nerve impulses came from, how are they made and where does the plant get its power to "wake up", turn around, and fall again asleep? Does the minimum of a light is enough for a plant to produce enough Adenosintriphosphat to wake up and does the plant consume this ATP to align or does another recipe is used for this and ATP is only for growing, getting bigger and producing seeds? --2A02:A312:6041:E900:F4BE:8393:F751:85B0 (talk) 01:21, 13 December 2020 (UTC)[reply]

It's plant hormones like auxin, as the above articles discuss. You know an adrenaline rush? That's hormones. Same principle, just different chemicals. The energy plants use is ultimately from light, which they capture via photosynthesis, store as chemical energy, then "burn" in their mitochondria to power processes such as synthesizing hormones, just as in your body. --47.152.93.24 (talk) 02:51, 13 December 2020 (UTC)[reply]
Don't most non-woody plants tend to lean toward the sun? ←Baseball Bugs What's up, Doc? carrots04:08, 13 December 2020 (UTC)[reply]
Our articles Heliotropism and Phototropism may shed some light on the question, but do not give a definive answer as to the number of species or proportion of all plants that exhibit them. {The poster formerly known as 87.81.230.195} 2.122.56.237 (talk) 04:42, 13 December 2020 (UTC)[reply]
Animals and plants alike use ATP for basically all processes that require energy. Plants use it for the transport of ions throughout the plant, which is used for rapid plant movement, but also for the twisting of leaf stems of slow-moving plants that can be made apparent by time-lapse photography. The physics underlying such movement is that of cells getting higher or lower turgor pressure due to water moving in and out of these cells by osmotic action in response to changes in the ion concentrations.  --Lambiam 13:33, 13 December 2020 (UTC)[reply]

Psychology

Decribe Maslow's hierarchy of needs stating how it can be applied to improve learning in Kenyan schools — Preceding unsigned comment added by 102.167.83.155 (talk) 08:26, 13 December 2020 (UTC)[reply]

Do your own homework. This will help: Maslow's hierarchy of needs.Graham Beards (talk) 08:56, 13 December 2020 (UTC)[reply]
And have a read of Wikipedia:Do your own homework. HiLo48 (talk) 08:57, 13 December 2020 (UTC)[reply]
In addition to various other no-nos about the ref desks, there should be one that says we will not answer "questions" which read like "demands". ←Baseball Bugs What's up, Doc? carrots09:14, 13 December 2020 (UTC)[reply]
That might involve telling the IP editor's teacher to not set homework questions that read like demands. HiLo48 (talk) 09:22, 13 December 2020 (UTC)[reply]
There is some sort of template which identifies a given IP as belonging to a school. Such a caution could be included in that template. ←Baseball Bugs What's up, Doc? carrots10:55, 13 December 2020 (UTC)[reply]
The OP's 'question' read like it was taken straight from the exercise set by a teacher/lecture. LongHairedFop (talk) 11:00, 13 December 2020 (UTC)[reply]
For sure. ←Baseball Bugs What's up, Doc? carrots11:31, 13 December 2020 (UTC)[reply]
Assuming Maslow's hierarchy of needs can be applied to improve learning in the schools of country X, then answering the "how" question requires a good understanding of country X and its educational system – unless the answer is embedded in the class notes of a school textbook and merely needs to be reproduced. If students are malnourished and lack energy, then surely a well-administered [9]school lunch program can offer some improvement. Developing the ability of critical thinking is hardly stimulated in many countries' educational systems, but is important for self-actualization – as well as for a country's vitality and resilience.  --Lambiam 13:04, 13 December 2020 (UTC)[reply]

Zebra

Zebra#Stripes currently says "Striping patterns are unique to an individual and heritable". This looks contradictory to me: if they are heritable, then they could not be unique to an individual, because an offspring would inherit the same pattern as the parent (and vice versa). Please clarify if anything, thanks. 212.180.235.46 (talk) 22:06, 13 December 2020 (UTC)[reply]

Compare "A human's facial features are unique to an individual and heritable." Heritable does not mean replicated exactly. HenryFlower 22:15, 13 December 2020 (UTC)[reply]
It seems kind of poorly worded. Presumably, what they are trying to say is that stripes are heritable, and striping patterns are unique to an individual. ←Baseball Bugs What's up, Doc? carrots22:47, 13 December 2020 (UTC)[reply]
Ok, that makes sense then. 212.180.235.46 (talk) 00:21, 14 December 2020 (UTC)[reply]
There is more to it than that having stripes is heritable. Some zebras have thin stripes, others thick stripes, and some thin stripes on some parts of their body but thick on other parts. These distributional patterns are heritable and characteristic for the zebra species. For example, the mountain zebra has thin stripes except on its hind quarters, where they are rather thick. The (threatened) Grévy's zebra has thin stripes almost everywhere, a bit thicker in its neck and one very thick stripe down the middle of its back, along the length and extending into the tail. The mountain zebra has transversal stripes there. A zebrologist can determine the species and often the subspecies purely from this distributional pattern. Zebras also recognize their conspecifics this way. There is still an immense variety of possible concrete, individual patterns that conform to the distribution of a specific distributional pattern, just like no two fingerprint arches from different fingers are exactly alike.  --Lambiam 11:32, 14 December 2020 (UTC)[reply]

December 14

How small could red green blue camera pixels get?

What shrinking levels are banned by laws of physics and what just need sufficiently advanced technology? Sagittarian Milky Way (talk) 22:55, 14 December 2020 (UTC)[reply]

Why do so many explosives feature the elements of life i.e. H, C, N, O?

For example, gunpowder (NaNO3, C, S); nitroglycerine C3H5N3O9; nitrocellulose (C6H9(NO2)O5)n; and ammonium nitrate NH4NO3.

Or, put another way, why are many explosives organic, as opposed to inorganic?

Our article on explosives says, "explosives are substances that contain a large amount of energy stored in chemical bonds" which seems to suggest that a lot of energy would required to break these bonds? --- Sandbh (talk) 23:10, 14 December 2020 (UTC)[reply]

It's because there are structural elements within those compounds, nitrate groups in particular, that will readily decompose into more stable forms such as gaseous nitrogen N2, and release a lot of heat when they do so. Organic explosives (e.g. TNT, RDX) tend to be preferred over inorganic explosives (e.g. metal azides) because they can be stored and detonated in a controlled manner, are less shock-sensitive, and are generally more reliable. Bonkeyballs (talk) 00:31, 15 December 2020 (UTC)[reply]