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Friction is the force resisting the relative lateral motion of two solid surfaces in contact, or a solid surface in contact with a fluid (e.g. air on an aircraft or water in a pipe). It is not a fundamental force, as it is derived from electromagnetic force between charged particles, including electrons, protons, atoms, and molecules, and so cannot be calculated from first principles, but instead must be found empirically. When contacting surfaces move relative to each other, the friction between the two surfaces converts kinetic energy into thermal energy, or heat. Friction between solid objects is often referred to as dry friction and between a solid and a gas or liquid as fluid friction. Friction between moving surfaces is called kinetic friction. Friction between non-moving surfaces is called static friction. Contrary to many popular explanations, sliding friction is caused not by surface roughness but by chemical bonding between the surfaces.^[1] Surface roughness and contact area, however, do affect sliding friction for micro- and nano-scale objects where surface area forces dominate inertial forces.^[2] Internal friction is the motion-resisting force between the surfaces of the particles making up the substance. Friction should not be confused with traction. Surface area does not affect friction significantly because as contact area increases, force per unit area decreases. However, in traction surface area is essential.

Coulomb friction, named after Charles-Augustin de Coulomb, is a model to describe friction forces. It is described by the equation

where

For surfaces at rest relative to each other µ = µ_s, where µ_s is the coefficient of static friction. This is usually larger than its kinetic counterpart. The Coulomb friction may take any value from zero up to F_f, and the direction of the frictional force against a surface is opposite to the motion that surface would experience in the absence of friction. Thus, in the static case, the frictional force is exactly what it must be in order to prevent motion between the surfaces; it balances the net force tending to cause such motion. In this case, rather than providing an estimate of the actual frictional force, the Coulomb approximation provides a threshold value for this force, above which motion would commence.

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