Friction is the force that resists the movement of one surface over another. When one surface is moving relative to the other, the friction is "kinetic" — sliding friction. Contrariwise, if the surfaces are not moving — or are at rest — relative to each other, the friction is static. For static friction, if the total applied force on an object is "F" and the resistive force from friction is "f," then there exists some coefficient, μs, such that f = μs × F. If F becomes greater than f, static friction gives way to sliding friction, and the mathematical expression becomes f = μk × F, where μk is the coefficient of kinetic, or sliding, friction.
Note that the equations for friction contain no terms readily identifiable with the causes of friction. This is because of the wide variance in phenomena that add to friction. These include surface interactions resulting from "adhesion," "plowing" and "asperity deformation." Adhesion refers to the component of sliding friction that results from the electrostatic attraction of atoms. Forces of an adhesive nature between two surfaces can be weak — as in the case of Teflon® coated, or oiled, surfaces — or quite strong, essentially infinite — in the case of powerful adhesives.
Two mostly-intact surfaces have imperfections — a roughness or harshness of surface — called asperities. These can interlock at least briefly. There are two mechanisms that still enable such surfaces to move in relation to each other, experiencing sliding friction, without coming to a halt. One of these is plastic deformation, whereby the obstruction temporarily is pushed aside. The other is plowing, which is where one surface feature plows away the imperfection of the other surface, much as a farmer’s plow digs away the dirt beneath its blade, allowing movement.
Once two surfaces at rest overcome the force of static friction, they engage in sliding friction. This remains the case as long as the surfaces are in touch and the force remains great enough to continue the action. For most real-world applications, the force of static friction just before movement begins is greater than that experienced during sliding friction. It has been found, however, that if surface imperfections are carefully minimized, the level of force that must be achieved to initiate sliding friction is about the same as that required to maintain it.
There are other forces at work that could be viewed as resembling sliding friction in some senses. For example, a magnetic field is capable of producing what may be considered a kind of "friction" in a dynamo. A small magnetic braking component results. This is usually categorized as "magnetic damping" rather than as sliding friction.
