# Coefficient of friction

The coefficient of friction is shown by $f = \mu F_{n}\,$. In that equation, $f$ is the frictional force, $\mu$ is the coefficient of friction, and $F_{n}\,$ is the normal force.
$\mu$ can be two different things. It can be $\mu_s$, or it can be $\mu_k$. $\mu_s$ is the coefficient of static friction. The coefficient of static friction is the friction force between two objects when neither of the objects is moving. $\mu_k$ is the coefficient of kinetic friction. The coefficient of kinetic friction is the force between two objects when one object is moving, or if two objects are moving against each other.