Meaning of Coefficient of Friction
All coefficients of friction are dimensionless scalar quantities; individual contributing factors resist full explanation or quantification. The fundamental force responsible for the greatest friction is also the one that allows the formation of chemical bonds: the electrostatic force.
Friction is the resistive force that produces heat generated by the movement of two contact 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 strictly predetermine from theoretical first principles. Mathematically, the expression for friction includes a single constant that incorporates all of the causal factors: a coefficient of friction (COF), symbolized by the Greek letter, ?.
The equation is simply written fx = ? x F, where fx defines the shape and size of the friction, while F is the perpendicular or “normal” force exerted by both surfaces, one on the other.
On initial consideration, it might appear that gravity is the source of 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 this is determined at a microscopic level by the electrical charges that tend to prevent movement by “binding” them together. Such bonding is a characteristic of adhesives used to cement two surfaces together.
That this is the case is well illustrated by the modern polymer, polytetrafluoroethylene (PTFE). Better known under its DuPont™ brand name, Teflon®, PTFE exhibits only very weakly attractive London dispersion electrostatic forces.
This gives PTFE a coefficient of friction among the known “top three”, approximately 0.05-0.10. If the coefficient of friction were a product of gravity, the chemistry of the surfaces would not matter, and the substances that 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 layer of gas between the surfaces, which decreases the apparent weight of the surface on top and eliminates manufacturing defects such as surface roughness.
The change in effective weight decreases the normal force, while the lack of defects changes the coefficient of friction; both mathematically reduce the resulting frictional force. Engineers have used the gas cap phenomenon to develop hovercraft to travel across land and water surfaces.