Friction force

When one body tries to move or moves over the surface of another, a friction force arises between the surfaces in contact and hinders the motion. Its causes are the roughness of the surfaces and the attraction between molecules. Friction is directed against the motion, along the surfaces. Static friction arises when we try to start moving a body: it equals the applied force in magnitude and is opposite, so the body stays still. As the applied force grows, static friction also grows up to a certain maximum value; beyond it the body begins to slide. The maximum static friction is usually slightly larger than sliding friction.