Friction is the resistive force acting between bodies that tends to oppose and damp out motion. Friction is usually distinguished as being either static friction (the frictional force opposing placing a body at rest into motion) and kinetic friction (the frictional force tending to slow a body in motion). In general, static friction is greater than kinetic friction.

The force due to kinetic friction is generally proportional to the applied force, so "a coefficient of kinetic fiction" is defined as the ratio of frictional force to the normal force on the body.

The study of friction is called tribology.

Bagnold Friction Law, Frictional Force, Kinetic Friction, Starting Friction, Static Friction