Definition:Friction/Coefficient

Definition
Let $B$ be a body at rest on a plane surface $S$ on which friction acts.

Let $N$ be the normal reaction of $S$ on $B$.

Let a force be applied to $B$ parallel to $S$.

Coefficient of Kinetic Friction
These coefficients of friction depend upon the materials out of which $B$ and $S$ are made.

It is usual for $\mu_s$ to be greater than $\mu_k$.