Section 1 of 6What happens when a car brakes
1Pedal pressed
- Pedal pressed The driver pushes the brake pedal and brake pads are forced against the discs or drums.
2Friction force
- Friction force Friction between pad and disc acts against the rotation of the wheel.
3Work done
- Work done The braking force does work, so the kinetic energy of the car falls and the brakes get hot.
4Vehicle stops
- Vehicle stops When all of the kinetic energy has been transferred to thermal energy the car is at rest.
Section 2 of 6Factors that increase braking distance
Higher speed
- Higher speed Kinetic energy is ½ × m × v2, so doubling the speed needs four times the work done by the brakes and about four times the distance.
Greater mass
- Greater mass A heavier vehicle has more kinetic energy at the same speed, so it needs a longer distance for the same braking force.
- A lorry takes longer to stop than a car.
Worn brakes
- Worn brakes Worn pads give a smaller braking force, so deceleration is smaller and the car travels further.
Worn tyres
- Worn tyres Low tread cannot clear water, so the contact with the road is poor and the tyres skid.
Wet or icy roads
- Wet or icy roads Less friction between tyre and road means the wheels lock or slip and the grip is lost.
Section 3 of 6Why distance is related to force
1
Work done by the brakes = braking force × braking distance.
2
All of the car's kinetic energy must be removed, so F × d = ½ × m × v2.
3
For a given braking force, braking distance increases with the square of speed.
4
For a given speed, a smaller braking force means a longer braking distance.
Section 4 of 6Dangers of large deceleration
| Problem | Consequence |
|---|---|
| Very strong braking force | Large deceleration, which can injure passengers or cause the car to skid |
| Brakes overheating | Brake fade; the brakes become less effective after repeated use |
| Loss of grip | Wheels lock, the car skids and the driver loses steering control |
| Wet roads in combination with worn tyres | Aquaplaning, where a water layer lifts the tyre off the road |
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Braking force | The force produced by the brakes that opposes the motion of the vehicle. |
| Friction | A contact force that opposes the relative motion of two surfaces. |
| Kinetic energy | The energy of a moving object, ½ × m × v2. |
| Deceleration | The rate at which a vehicle slows down. |
| Brake fade | Loss of braking effectiveness as the brakes overheat. |
Section 6 of 6Exam technique
1
Saying that a car with greater mass will always stop in the same distance. More mass means more kinetic energy, so the braking distance is longer.
2
Claiming braking distance is proportional to speed. It depends on speed squared.
3
Mixing up thinking and braking factors. Weather, tyres and brakes affect braking distance only.
4
Forgetting to say where the energy goes: kinetic energy is transferred to thermal energy in the brakes.