Slipping and Sliding

A round object rolling on any surface with a nonzero coefficient of friction ($\mu>0$) always tends towards pure rolling — the friction responsible for enabling and maintaining this is called rolling friction. In pure rolling, there is, by definition, no relative sliding motion between the point of contact and the surface at all. But whenever the rolling object suddenly speeds up or slows down (or, indeed, whenever it is not yet in pure rolling to begin with, e.g. just after starting from rest, or just after landing), the balance $v_{CM}=R\omega$ can be disturbed, producing either sliding or slipping.

Sliding is the case $v_{CM}>R\omega$ (equivalently, $v_{TRANS}>v_{ROT}$) — translation exceeds rotation. This happens, for example, when sudden brakes are applied to a moving vehicle, or when a vehicle enters a slippery patch of road. Here, the point of contact has more translational velocity than rotational velocity, giving a net forward resultant velocity there relative to the ground; the kinetic frictional force $f_k$, always opposing this relative motion, therefore acts backward at the contact point. This friction reduces the (excess) translational velocity and increases the rotational velocity, driving the two towards equality and re-establishing pure rolling. Sliding is sometimes also called forward slipping. …