Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other.
When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energy into heat. This property can have dramatic consequences, as illustrated by the use of friction created by rubbing pieces of wood together to start a fire. Kinetic energy is converted to heat whenever motion with friction occurs, for example when a viscous fluid is stirred. Read more on – Wikipedia
Video on Friction
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