Friction in Solids

Whenever the surface of one body slides — or tends to slide — over the surface of another, each body exerts a tangential opposing force on the other, directed opposite to the relative motion (or impending relative motion) between them. This mutual, motion-opposing mechanical force between two solid surfaces in contact is called frictional force, and the general property responsible for it — the resistance to relative motion between two surfaces in contact — is called friction. (This chapter deals only with friction between solid surfaces; friction between a solid and a fluid, such as a boat moving through water, follows related but distinct principles.)

Friction is an everyday, easily observed phenomenon: an object resting on any ordinary surface does not budge under a small applied force, because some opposing force resists that motion; even a ball rolling on a playground eventually comes to rest, purely because of an opposing force; the tread patterns on the soles of shoes exist specifically to generate enough opposing force to prevent slipping while walking; and anyone who has tried to walk quickly across a wet, soapy floor knows exactly what happens when that opposing force is largely absent. Starting any relative motion between two surfaces in contact requires a certain minimum applied force, and once motion has begun, it continues to be resisted by a (generally smaller, but still present) opposing force throughout. …