Friction is the force resisting the relative motion of two surfaces in contact or a 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 forces between atoms and electrons, 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 objects converts kinetic energy into thermal energy, or heat. Friction between solid objects is often referred to as dry friction or sliding friction and between a solid and a gas or liquid as fluid friction. Both of these types of friction are called kinetic friction. Contrary to many popular explanations, sliding friction is caused not by surface roughness but by chemical bonding between the surfaces. Surface roughness and contact area, however, do affect sliding friction for micro- and nano-scale objects where surface area forces dominate inertial forces. 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.

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