Breaking Force (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Work done by brakes

• When a braking force is applied to a vehicle, the friction force between the brakes and wheels does work

• This transfers energy from the vehicle's kinetic store, reducing its speed

• The temperature of the brakes increases as energy is transferred to the thermal store

• The greater the speed of a vehicle, the greater the braking force required to stop it in a given distance

• The greater the braking force, the greater the deceleration of the vehicle

• Large decelerations can be dangerous:

  • Brakes may overheat, making them less effective

  • The driver may lose control of the vehicle

Work done by brakes (Higher Tier Only)

Higher Tier Only

• The work done by the brakes when a vehicle slows to a halt is:

work done by the brakes = braking force x braking distance

• This is equal to the kinetic energy of the car:

kinetic energy of the car = x mass x velocity²

• This means that the single equation to show the work done by the brakes when a vehicle slows to a halt is

braking force x braking distance = x mass x velocity²

braking force x braking distance = mv²

• This equation shows that:

  • The work done is the transfer of kinetic energy

  • The braking distance is proportional to the speed squared (if the speed is doubled, the distance increases 4 times)

• We can use this equation to estimate the decelerating forces required for a typical vehicle moving at everyday speeds