Kinetic Friction

When two surfaces slide past each other, kinetic friction opposes their relative motion. Unlike static friction, kinetic friction has a constant magnitude:

$$f_k = \mu_k N$$

Here $\mu_k$ is the coefficient of kinetic friction (a dimensionless constant depending on the surfaces) and $N$ is the normal force magnitude. Importantly, $\mu_k < \mu_s$ (the coefficient of static friction). This means it's easier to keep an object sliding than to start it moving.

The direction of kinetic friction is always opposite to the object's velocity relative to the surface. If an object slides to the right, kinetic friction points to the left. This causes sliding objects to decelerate unless other forces maintain their motion.