# Are coefficients of friction invariant by switching the materials?

I don't have the ability to make the experiment to answer these questions :

Let $A$ and $B$ be two materials. Is the static coefficient of friction of $A$ on $B$ equal to the static coefficient of friction of $B$ on $A$ ? I ask the same question for the sliding coefficient of friction.

• It seems like it should have the same value. Changing which object is sliding is equivalent to changing your frame of reference from being attached to the "fixed" object (lab frame) to being attached to the "moving" object. Since everything has to be behave properly under a change of frame, the friction coefficient should be the same. Jul 2 '15 at 19:18
• That being said, sliding (eg.) a milled steel block on a cold-rolled aluminum surface is NOT the same as sliding a milled aluminum block on a rolled steel surface. The surface finishes will be different, so the friction can very well be different. Jul 2 '15 at 19:19

The friction equation tells us that $F_F=\mu*F_N$.
$F_N$ is the normal force, or the force that each of the materials applies on each other.