A brake is a mechanical device that inhibits motion, slowing or stopping a moving object or preventing its movement. Usually, brakes use friction between two surfaces pressed together to convert the kinetic energy of the moving object into heat, though other energy conversion methods may be employed.
For example, regenerative braking converts much of the energy to electrical energy, which may be stored later. Other methods convert kinetic energy into potential energy in such stored forms as pressurized air or oil. Eddy current brakes use magnetic fields to convert kinetic energy into electric current in the brake disc, fin, or rail converted into heat.
Still, other braking methods transform kinetic energy into different forms, for example, by transferring the power to a rotating flywheel. Brakes are generally applied to rotating axles or wheels but may also take other conditions, such as the surface of a moving fluid (flaps deployed into water or air). Some vehicles use a combination of braking mechanisms, such as drag racing cars with both wheel brakes and a parachute or airplanes with both wheel brakes and drag flaps raised into the air during landing. Since kinetic energy increases quadratically with velocity (), an object moving at ten m/s has 100 times as much power as one of the same mass moving at one m/s. Consequently, when braking at the traction limit, the theoretical braking distance is 100 times as long. In practice, fast vehicles usually have significant air drag, and energy lost to air drag rises quickly with speed.
Almost all wheeled vehicles have a brake of some sort. Even baggage and shopping carts may have them on a moving ramp. Most fixed-wing aircraft are fitted with wheel brakes on the undercarriage. Some aircraft also feature air brakes designed to reduce their speed in flight. Notable examples include gliders and some World War II-era aircraft, primarily some fighter aircraft and many dive bombers of the era. These allow the aircraft to maintain a safe speed in a steep descent.
The Saab B 17 dive bomber and Vought F4U Corsair fighter used the deployed undercarriage as an air brake. Friction brakes on automobiles store braking heat in the drum brake or disc brake while braking, then conduct it to the air gradually. When traveling downhill, some vehicles can use their engines to brake. When the brake pedal of a modern car with hydraulic brakes is pushed, a piston moves the brake pad against the brake disc, slowing the wheel down.
The brake drum is similar as the cylinder pushes the brake shoes against the drum and slows the wheel down.