Friction is the force that opposes the relative motion or tendency of such motion of two surfaces in contact.
See also:
It is also the contact of two objects creating static electricity.
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 sensitive energy, or heat (atomic vibrations).
Friction between solid objects and fluids (gases or liquids) is called fluid friction..
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