Friction

Friction accounts for resistance forces that arise at contacting surfaces and oppose relative sliding or rolling. Unlike an ideal smooth contact, a real contact can transmit tangential forces.

Sliding friction

Dry friction acts against the tendency of relative sliding. During static contact its magnitude adjusts to the applied loading up to a limiting value related to the normal reaction and the coefficient of friction.

Equilibrium with friction

In equilibrium problems, the direction of friction is determined from the impending or possible relative motion. It is important to distinguish ordinary equilibrium from limiting equilibrium, where slipping is about to begin.

Rolling resistance

Rolling resistance is associated with deformation in the contact region and displacement of the resultant contact reaction. Its mechanical model differs from the Coulomb model of dry sliding friction.