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A brake is a device for slowing or stopping the motion of a machine, and to keep it from starting to move. The kinetic energy lost by the moving part is usually translated to heat by friction. Alternatively, in regenerative braking, the energy is recovered and stored in a flywheel, capacitor or other device for later use.

Brakes of some description are fitted to most wheeled vehicles, including automobiles of all kinds, trains, motorcycles, and bicycles. Baggage and shopping carts may have them for use on a moving ramp. Some aeroplanes are fitted with wheel brakes on their landing wheels. Some also feature air brakes designed to slow them down in flight (notable examples include some WWII-era fighter aircraft which used "dive brakes" to allow the planes to maintain a safe speed in a steep descent).

Deceleration of cars etc., and avoiding acceleration when going downhill, is also achieved by using a low gear.

Types of brake

See also Bicycle brake systems