A brake is a device for slowing the speed of a vehicle or machine. Braking can be accomplished by a mechanical, hydraulic (liquid), or pneumatic (air) system that presses a non-rotating part into contact with a rotating part, so that friction stops the motion. In a car, the non-rotating part is called a shoe or pad, and the rotating part is a drum or disk attached to a wheel. Some vehicles use electromagnetic effects to oppose the motion and cause braking. A "power" brake utilizes a vacuum system.