Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding or grinding against each other.^[2][3] Types of friction include dry, fluid, lubricated, skin, and internal – an incomplete list. The study of the processes involved is called tribology, and has a history of more than 2,000 years.^[4]

Friction can have dramatic consequences, as illustrated by the use of friction created by rubbing pieces of wood together to start a fire. Another important consequence of many types of friction can be wear, which may lead to performance degradation or damage to components. It is known that frictional energy losses account for about 20% of the total energy expenditure of the world.^[5][4]

There are many different contributors to the retarding force in friction, ranging from asperity deformation to the generation of charges and changes in local structure. When two bodies in contact move relative to each other, due to these various contributors some mechanical energy is transformed to heat, the free energy of structural changes, and other types of dissipation. The total dissipated energy per unit distance moved is the retarding frictional force, F=E/d. The complexity of the interactions involved makes the calculation of friction from first principles difficult, and it is often easier to use empirical methods for analysis and the development of theory.^[3][2]