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Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding or grinding against each other. 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. 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 cons...