Friction is the force that opposes the relative motion or tendency of such motion of two surfaces in contact. It is not, however, a fundamental force, as it originates from the electromagnetic forces and exchange force between atoms. In situations where the surfaces in contact are moving relative to each other, the friction between the two objects converts kinetic energy into heat (atomic vibrations). Friction between solid objects and fluids (gases or liquids) is called fluid friction. See also aerodynamics and hydrodynamics.

Friction is an extremely important force - it propels automobiles and other ground transport and holds nails, screws and nuts, along with many other uses.

Equations

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The classical approximation of the force of friction known as Coulomb friction (named after Charles-Augustin de Coulomb) is expressed as: $F\_f\; =\; \backslash mu\; N$, where $\backslash mu$ is the coefficient of friction, $N$ is the force normal to the contact surface, and $F\_f$ is the force exerted by friction. This force is exerted in the direction opposite the object's motion.

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