User:Aarchiba/Friction

Friction is the force between two surfaces which opposes their relative sliding 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.

Friction-like resistance can occur in other contexts; fluids experience viscosity, solid objects moving through fluids experience fluid friction, and sound waves in solids, liquids, and gases lose energy as they propagate.

Simple model of friction
Couldomb friction divides friction into two kinds: kinetic friction and static friction. Kineticgfd friction dis proportional to the normal force N:


 * $$F = \mu_k N$$

The number $$\mu_k$$ is called the coefficient of kinetic friction, and it is assumed to be a property of the combination of surfaces. The frictional force in this model is independent of the area of contact and the velocity of the sliding (within a moderate range of velocities).

Static friction occurs when there is no sliding. In such a situation the model states that the frictional force will be exactly as large as needed to prevent sliding, up to a certain maximum force $$F_m$$, which is given by:


 * $$F_m = \mu_s N$$

The number $$\mu_s$$ is called the coefficient of static friction, and is again assumed to be a property of the combination of surfaces. Once again, this model predicts the frictional force should be independent of the area of contact.

The independence of the friction force on the area of contact is, at first glance, the most puzzling aspect. One simple model to describe it is as follows. Surfaces are known to be very rough on a microscopic scale. So when two surfaces are apparently in contact over some large area, only the tips of the rough projections (called asperities) are actually in contact, so the "real contact area" is much smaller than the apparent contact area. When a normal force N is applied, the fraction of the area that is actually in contact increases in proportion to the pressure, that is, the normal force divided by the apparent contact area. The resulting frictional force is then independent of the apparent contact area, as increasing the apparent area decreases the pressure. (Tipler 1991, p. 108)

Practical measures to control friction
Engineers generally need to plan for friction in several 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. They may need to provide controlled slipping to dissipate energy, for example in automobile brake drums. Finally, a system may need to have consistent frictional behaviour in order to function smoothly, for example in knitting machines. In each case the measures taken are different.