# Definition:Friction/Coefficient/Kinetic

## 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 $S$

Let $F$ be the magnitude of that force needed to keep $B$ moving at a constant velocity.

Then the coefficient of kinetic friction is defined and denoted:

$\mu_k = \dfrac F N$

## Linguistic Note

The word friction comes from the Latin word for rub.