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Friction is the force between two surfaces which opposes their relative motion. The microscopic physics which produce this force are not well-understood, and its behaviour is highly complex, both theoretically and experimentally. The study of friction is intimately connected to the study of lubrication and of wear; the discipline is known as tribology.

Theories of friction

Coulomb friction

A simple model of friction that is in common use, particularly in college physics courses, divides frictional forces into static friction and kinetic friction. Static friction occurs when the two surfaces are not in relative motion, and it provides a force sufficient to prevent the surfaces from moving relative to one another, up to a certain maximum frictional force. This maximum force is (in this simple approximation) proportional to the force pressing the objects together; the coefficient of proportionality is called the coefficient of static friction. Kinetic friction occurs when surfaces are in relative motion, and it provides a force that opposes the relative motion; this force is proportional to the force pressing the two objects together, and the coefficient of proportionality is called the coefficient of kinetic friction.

Adhesion

Rolling resistance

Practical measures to control friction

Engineers generally need to plan for friction in three different contexts. They may need to reduce friction as much as possible, for example by lubrication of an axle. They may need to provide a very large amount of friction to prevent slip, for example in automobile tires. Or they may need to provide controlled slipping, for example in automobile brake drums.

See also

Bibliography

  • Ludema, Kenneth C., Friction, Wear, Lubrication: A Textbook in Tribology, CRC Press 1996, ISBN 0-8493-2685-0
  • Tipler, Paul A., Physics for Scientists and Engineers, Third Edition, Extended Version, Worth Publishers 1991, ISBN 0-87901-432-6
  • Feynman, Leighton, Sands, The Feynman Lectures on Physics, Volume I, Addison-Wesley Publishing Company, 1963, ISBN 0-201-02010-6-H
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