Coefficient of kinetic friction

Definition

Given two surfaces, the coefficient of kinetic friction or coefficient of dynamic friction between them is the constant $\mu _{k}$ such that, for small speeds of slipping of two bodies with these two as the surfaces of contact, the kinetic friction between the two bodies is $\mu _{k}$ times the normal force between the two bodies.

The coefficient of kinetic friction is usually slightly, but not much, smaller than the limiting coefficient of static friction.

Units and dimensions

The coefficient of kinetic friction is a dimensionless scalar and does not need units.