Easiest Way to understanding coefficient of friction? So I am having issues understanding how to find the coefficient of friction, I should have understood this couple chapters ago but I couldn't. I am not understanding the difference between the static coefficient and the kinetic coefficient (I know static is not moving and kinetic is moving). Maybe you can help me understand it better than my teacher explained?
 A: The coefficient of friction is just the proportionality constant between the normal force and the friction force magnitudes between two surfaces: $f=\mu N$
Kinetic Friction
This is, in my opinion, the easier one to understand. If the two surfaces are moving relative to each other, then kinetic friction will be the force opposing this relative motion. The magnitude of the friction force is given by $f=\mu N$, where $\mu$ is our coefficient of friction, and $N$ is the magnitude of the normal force between the two surfaces.
Static Friction
This is a bit different than kinetic friction. It is at play when two surfaces are at rest relative to each other, but there is still some external force being applied that is attempting to cause the surfaces to slide past each other.
In this case, instead of saying $f=\mu N$, we have $f\leq\mu N$. What this means is that we are given the largest the static friction force can be before it fails and the surfaces start sliding past each other. Before this is reached, the static friction force is whatever it needs to be to stop the object from moving (imagine a string that will hold an object in place until you pull on it too hard).
A point to be made. This is an extremely simplified model of friction. It does not describe what happens on smaller scales; it is a macroscopic model. But it still does a good job at describing friction between many types of surfaces.
