Friction can be kinetic or static, with coefficients μs and μk respectively. Friction is caused by surface interactions such as adhesion and asperity deformation. Once static friction is overcome, sliding friction occurs. Other forces, such as magnetic damping, can also be considered similar to sliding friction.
Friction is the force that resists the movement of one surface against another. When one surface moves relative to another, the friction is “kinetic” – sliding friction. Conversely, if the surfaces are not moving – or are stationary – relative to each other, the friction is static. For static friction, if the total force applied on an object is “F” and the force resisting friction is “f”, then there is a 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 friction equations do not contain terms that are easily identifiable with the causes of the friction. This is due to the wide variation in phenomena that increase friction. These include surface interactions resulting from “adhesion”, “ploughing” and “asperity deformation”. Adhesion refers to the component of sliding friction that results from the electrostatic attraction of atoms. The adhesive forces 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 strong adhesives.
Two mostly intact surfaces have imperfections — a roughness or hardness of the surface — called asperities. These can interlock at least briefly. There are two mechanisms that still allow such surfaces to move relative to each other, experiencing sliding friction, without stopping. One of these is plastic deformation, whereby the obstruction is temporarily pushed aside. The other is ploughing, which is where one surface feature sweeps away the imperfection of the other surface, much like a farmer’s plow digs away the earth beneath its blade, allowing for movement.
Once two surfaces at rest overcome the static friction force, they engage in sliding friction. This remains the case as long as the surfaces are in contact and the force remains large enough to continue the action. For most real-world applications, the static friction force just before motion 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 approximately the same as that required to maintain it.
There are other forces at work that could be seen as similar to sliding friction in some sense. For example, a magnetic field can produce what can be considered a kind of “friction” in a dynamo. A small magnetic braking component results. This is usually classified as “magnetic damping” rather than sliding friction.
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