Question

If `F_N = 25N` and `mu_k=.13`, what is kinetic friction?

Answer 1

The kinetic friction, `F_k` is just the product of the normal force and the coefficient of friction: `F_k=F_N*mu_k=25*.13 =3.25 N`

Explanation:

That's pretty much all there is to it.

A friction coefficient is just a dimensionless number (that is, it has no units) that relates the frictional force to the normal force. Typically there is a different number for static friction - when the two objects are not moving relative to each other - and kinetic friction - which the two objects are sliding over one another. The kinetic friction is usually smaller: once a car tire starts sliding it will usually keep sliding more easily, because the static friction has been overcome and the kinetic friction is smaller.