Kinetic friction
Kinetic friction
We speak of sliding friction or kinetic friction when the object does move relative to the surface.
The magnitude of the force of sliding (or kinetic) friction is proportional to the normal force between the surface and the object and depends on the roughness of the surfaces:
\[F_{\mathrm{k}} = \mu F_{\mathrm{N}},\]
where \(\mu\) is the coefficient of kinetic friction, which reflects the roughness of the surfaces and depends on the material and coarseness of the surfaces themselves.
The vector of the force of kinetic friction, \(\mathbf{F}_{\mathrm{k}},\) is always directed opposite to the direction of the motion.