2.1.4Forces, Space and Radioactivity
Thinking, Braking and Stopping Distances
5 min read 6 parts Lesson 5 of 41 Unit quiz
Section 1 of 6Stopping distance
| Distance | Meaning | Main factors |
|---|---|---|
| Thinking distance | Distance travelled during the driver's reaction time | Speed of the vehicle; reaction time of the driver |
| Braking distance | Distance travelled while the brakes are slowing the vehicle | Speed; braking force; road and tyre condition; mass of the vehicle |
| Stopping distance | stopping distance = thinking distance + braking distance | All of the above |
Section 2 of 6What happens when the brakes are applied
1Friction force
- Friction force The brake pads press on the wheels, producing a frictional braking force.
2Work is done
- Work is done The braking force does work, transferring energy from the car's kinetic store.
3Brakes heat up
- Brakes heat up The energy goes to the thermal store of the brakes, so they get hot.
4Car slows
- Car slows With less kinetic energy, the car decelerates and eventually stops.
Section 3 of 6Factors affecting the distances
Reaction time
- Reaction time The average human reaction time is about 0.25 s.
- Tiredness, distractions such as a mobile phone, and alcohol or drugs make it longer, so thinking distance increases.
Speed
- Speed Thinking distance is directly proportional to speed.
- Braking distance increases much faster, because kinetic energy depends on the square of the speed.
Road and vehicle condition
- Road and vehicle condition Wet or icy roads, worn tyres and worn brakes reduce friction,
- so the braking force is smaller and the braking distance is longer.
Mass
- Mass A heavier vehicle has more kinetic energy at the same speed, so it needs more work to stop.
Section 4 of 6Why large decelerations are dangerous
Stopping from high speed
Greater force needed
- Greater force needed To stop in a given distance from a higher speed, a larger braking force is needed.
- By Newton's second law, a larger resultant force gives a larger deceleration.
Overheating
- Overheating A bigger energy transfer can make the brakes too hot, so they work less well.
Loss of control
- Loss of control Very large decelerations can make the tyres skid, so the driver may lose control of the vehicle.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Reaction time | The time between seeing a hazard and starting to respond to it. |
| Thinking distance | The distance a vehicle travels during the driver's reaction time. |
| Braking distance | The distance a vehicle travels once the brakes are applied until it stops. |
| Stopping distance | The total of the thinking distance and the braking distance. |
| Braking force | The frictional force between the brakes and wheels that slows the vehicle. |
Section 6 of 6Exam technique
1
Saying that poor road conditions increase thinking distance: they change the braking distance.
2
Saying that tiredness affects braking distance: it increases reaction time, so it affects thinking distance.
3
Writing that stopping distance is the braking distance alone.
4
Describing the energy as destroyedkinetic energy is transferred to thermal energy in the brakes.
5
Assuming doubling the speed doubles the braking distance: it increases by about four times.