Mr. Rogers' AP Physics C: Mechanics - Description of Static and Dynamic Friction

Static friction: With static friction there is no sliding, the friction force is exactly equal to the parallel force.

Transition point: At the transition point, the equation shown at right can calculate the highest possible static friction and this is directly proportional to normal force, but only at the transition point. Otherwise, static friction is determined solely by the parallel force.

Dynamic friction: When sliding occurs, the friction force remains constant. Typically dynamic friction is lower than static but never higher. Dynamic or sliding friction is influenced only by the normal force.

Note: there are three distinctly different equations. According to all three equations, contact area has no influence on friction.

Graph of Friction vs. Parallel Force