Friction is a force that resists the relative motion between two objects. The simplest form is dry friction, which is equal to

F_F = \mu F_N

μ is the coefficient of friction and FN is the normal force. The coefficient of friction is experimentally determined and is specific to the two materials in contact. In many materials, the coefficients of kinetic friction (when the objects are sliding) and static friction (when the objects are stationary relative to each other) are different.

Viscosity, skin drag, and rolling resistance are also forms of friction.