Section 1 of 5How Braking Works
Braking removes the kinetic energy of a vehicle by doing work against the motion.
1
The brakes apply a force that does work on the vehicle, removing its kinetic energy.
2
Work done = force × distance, so for a fixed braking force the braking distance depends on the kinetic energy of the vehicle.
3
Kinetic energy is ½mv², so doubling the speed quadruples the kinetic energy that must be removed.
4
A faster vehicle therefore travels much further before it stops, even with the same braking force.
5
Braking distance is measured from the point where the brakes are applied until the vehicle stops.
Section 2 of 5Factors That Change Braking Distance
Braking distance depends on the speed and mass of the vehicle, its brakes and tyres, and the condition of the road.
Speed
- Speed Braking distance is proportional to the square of the speed, so doubling the speed increases braking distance by a factor of four.
Mass
- Mass A heavier vehicle has more kinetic energy at the same speed, so it needs a longer braking distance for the same braking force.
Brakes and tyres
- Brakes and tyres Worn brakes or tyres with shallow tread reduce the friction available, so the braking force is smaller and the distance longer.
Road conditions
- Road conditions Wet, icy or leaf-covered roads reduce friction between the tyres and the road, which lengthens the braking distance.
| Factor | Effect on braking distance | Reason |
|---|---|---|
| Speed doubled | Increases by a factor of 4 | Kinetic energy is proportional to speed squared |
| Mass doubled | Doubles, at the same speed | Kinetic energy is proportional to mass |
| Wet road | Increases | Less friction, so a smaller braking force |
| Worn tyres | Increases | Less grip, so a smaller braking force |
Section 3 of 5Safety Implications
Safety Implications
Thinking distance
- Thinking distance This is the distance travelled during the driver's reaction time, and it increases with speed, tiredness and distraction.
Stopping distance
- Stopping distance Stopping distance is the thinking distance plus the braking distance, so it becomes much larger at high speed.
Reducing risk
- Reducing risk Drivers should keep larger gaps, slow down in poor conditions and make sure brakes and tyres are in good condition.
Section 4 of 5Worked Example
Question
- A car has a braking distance of 20 m when travelling at 20 m/s, with the same braking force on a dry road.
- Estimate its braking distance at 40 m/s.
- Braking distance is proportional to speed squared
- Ratio of speeds = 40 ÷ 20 = 2
- Ratio of braking distances = 2² = 4
- New braking distance = 4 × 20 = 80 m
Answer The braking distance at 40 m/s is about 80 m, assuming the braking force stays the same.
Section 5 of 5Exam Checklist & Tips
5.8.5 – Key Points for the Exam
- Braking distance The distance a vehicle travels while its brakes are applied, until it stops.
- Kinetic energy The energy an object has because of its motion, calculated as ½mv².
- Thinking distance The distance a vehicle travels during the driver's reaction time.
- Stopping distance The sum of thinking distance and braking distance.
- Friction The force between surfaces that opposes motion and allows brakes and tyres to slow a vehicle.
Common Mistakes
- Stating that braking distance is directly proportional to speed;
- it is proportional to the square of the speed.
- Confusing thinking distance with braking distance;
- thinking distance is the reaction time, while braking starts only when the brakes are applied.
- Forgetting that the same braking force is assumed when comparing two situations.
- Claiming that a heavier car has the same braking distance as a lighter car at the same speed.