Friction hinders perpetual motion by transforming kinetic energy into heat. But you already knew that...otherwise, why do you rub your hands to warm them?

Friction is also the resistance one feels when one drags an object on the ground, and that makes it possible to walk, turn with our cars or hold a cup of tea. This odd force, which always resists movement, is caused by the interaction between the molecules on the two surfaces that rub against each other.

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