So, we have seen that there are 2 kinds of friction: static and kinetic. Static friction occurs when an object isn't moving even though a force is being applied. Dynamic occurs when the frictional force isn't strong enough to completely stop the body from moving.

The formula to calculate the maximum frictional force is \(F_{\text{static}} = \mu_s N, \, F_{\text{kinetic}} = \mu_k N\). Here, \(\mu\) (pronounced mew) is called the co-efficient of static \(\mu_s\) or kinetic \(\mu_k\) friction.

\(\mu\) is constant for a given surface. The table here lists \(\mu\)'s for a variety of surfaces. You can see that the rougher surfaces have a higher \(\mu\).