Brake

Brake, device for slowing or halting motion, usually by conversion of kinetic energy into heat energy via the medium of friction. Most airplane, automobile, and railroad brakes are mechanical drum or disk brakes, applying friction directly to some portion of the wheel or of a drum rotating with the wheel. Mechanically operated brakes cannot always be used, as when a single control must operate on a number of wheels simultaneously and equally. Hydraulic, vacuum, and air brakes, which use different mediums to distribute the pressure, may be used in combination with mechanical brakes.