Friction is a surface force that opposes relative motion. The frictional force is directly related to the normal force that acts to keep two solid objects separated at the point of contact. There are two broad classifications of frictional forces: static friction and kinetic friction.

Static friction force

The static friction force (${\displaystyle F_{\mathrm {sf} }}$) will exactly oppose forces applied to an object parallel to a surface contact up to the limit specified by the coefficient of static friction (${\displaystyle \mu _{\mathrm {sf} }}$) multiplied by the normal force (${\displaystyle F_{N}}$). In other words, the magnitude of the static friction force satisfies the inequality:

${\displaystyle 0\leq F_{\mathrm {sf} }\leq \mu _{\mathrm {sf} }F_{\mathrm {N} }.}$

Kinetic friction force

The kinetic friction force (${\displaystyle F_{\mathrm {kf} }}$) is independent of both the forces applied and the movement of the object. Thus, the magnitude of the force equals:

${\displaystyle F_{\mathrm {kf} }=\mu _{\mathrm {kf} }F_{\mathrm {N} },}$

where ${\displaystyle \mu _{\mathrm {kf} }}$ is the coefficient of kinetic friction. For most surface interfaces, the coefficient of kinetic friction is less than the coefficient of static friction.