What is brake?

A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is
used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often
accomplished by means of friction.
Most brakes commonly use friction between two surfaces pressed together to convert the kinetic
energy of the moving object into heat, though other methods of energy conversion may be employed. For example, regenerative braking converts much of the energy to electrical energy, which may be stored for
later use. Other methods convert kinetic energy into potential energy in such stored forms as pressurized air or pressurized oil. Eddy current brakes use magnetic fields to convert kinetic energy into electric current in the brake disc, fin, or rail, which is converted into heat. Still other braking methods even transform kinetic energy into different forms, for
example by transferring the energy to a rotating flywheel.
Brakes are generally applied to rotating axles or wheels, but may also take other forms 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 ( K m v2 2 ), an object moving at 10 m/s has
100 times as much energy as one of the same mass
moving at 1 m/s, and consequently the theoretical
braking distance, when braking at the traction limit, 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 carts and shopping carts may
have them for use 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 vehicle with
hydraulic brakes is pushed against the master
cylinder, ultimately a piston pushes the brake pad
against the brake disc which slows the wheel down.
On the brake drum it is similar as the cylinder pushes
the brake shoes against the drum which also slows
the wheel down.

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