One of the simpler characteristics of sliding friction is that it is parallel to the contact surfaces between systems, and is always in a direction that opposes the motion or attempted motion of the systems relative to each other. If two systems are in contact and moving relative to one another, then the friction between them is called kinetic friction. For example, kinetic friction slows a hockey puck sliding on ice.

However, if two systems are in contact and are stationary relative to one another, then the friction between them is called static friction. The static friction is usually greater than the kinetic friction between two objects. For example, if a person is trying to slide a heavy crate across a concrete floor, they may be pushing very hard but the crate does not move at all. This means that the static friction responds to what we do; it increases to be equal to the push and acts in the opposite direction. If the person pushes hard enough, and the crate starts to move, this is the moment when static friction gives way to kinetic friction. Once in motion, it is easier to keep an object in motion than to get the motion started, indicating that the kinetic frictional force is less than the static frictional force.

This text is adapted from Openstax, University Physics Volume 1, Section 6.2: Friction.