Coeff. of friction?

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Friction is the heat-producing drag force generated by the movement of two touching surfaces against each other. It is indirectly the product of one of the four known fundamental forces. The friction of a system is impossible to rigorously predetermine from first theoretical principles. Mathematically, the expression for friction includes a single constant that incorporates all causal factors: a coefficient of friction (COF), symbolized by the Greek letter, μ. The equation is simply written fx = μxF, where fx defines the shape and extent of friction, while F is the perpendicular or “normal” force exerted by both surfaces on each other.

All coefficients of friction are dimensionless scalar quantities; individually contributing factors resist full explanation or quantification. The fundamental force responsible for most friction is also the one that allows chemical bonds to form: the electrostatic force. At first glance it might appear that gravity is the source of the friction, since the downward force due to gravity is the source of the variable F. In reality, however, the coefficient is a measure of the “stickiness” between the two surfaces, and that is determined at a microscopic level by the electric charges that tend to impede the movement by “binding” them together. Such a bond is a feature of adhesives used to cement two surfaces together.

That this is the case is well illustrated by the modern polymer, polytetrafluoroethylene (PTFE). Better known by the brand name DuPont, Teflon®, PTFE exhibits only very attractive London dispersion electrostatic forces. This gives PTFE a coefficient of friction in the known “top three” – around 0.05-0.10. If the coefficient of friction were the product of gravity, the chemistry of the surfaces would not matter and these substances would not be as economically important as they are.

There are ways to reduce friction without, in a sense, changing the materials the surfaces are made of. The coefficient of friction for a system can be effectively reduced by providing a thin film of lubricant. Alternatively, it is often possible to reduce friction by inserting a blanket of gas between the surfaces, which reduces the apparent weight of the overlying surface and eliminates manufacturing defects, such as surface roughness. The variation of the effective weight lowers the normal force, while the absence of defects modifies the coefficient of friction; both mathematically lower the resulting frictional force. Engineers have used the gas film phenomenon to develop hovercraft to travel on both land and water surfaces.

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