Stopping distance — AQA GCSE Physics
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Stopping distance explained
The two parts of the stopping distance grow differently with speed.
Read the full explanation
Thinking distance is speed × reaction time, so it is directly proportional to speed: at a reaction time of 0.6 s, doubling the speed from 10 m/s to 20 m/s doubles the thinking distance from 6 m to 12 m. Braking distance comes from energy. The kinetic energy to be removed is ½ m v², and the work done by the brakes is braking force × braking distance, so with a fixed braking force the braking distance goes with the square of the speed. Doubling the speed from 10 m/s to 20 m/s takes the braking distance from 6 m to 24 m, four times as far. The stopping distance therefore rises far more steeply than the speed.
Your focus
- Show with figures from a table that thinking distance is proportional to speed but braking distance is not.
- Explain, using ½ m v² and work done, why braking distance quadruples when the speed doubles.
- Evaluate a student's claim that both thinking and braking distance are proportional to speed.
Stopping distance exam tips
Marking Points
- one mark for stating that thinking distance is directly proportional to speed
- one mark for supporting that with figures, such as 6 m at 10 m/s rising to 12 m at 20 m/s
- one mark for stating that braking distance is not proportional to speed but increases more steeply
- one mark for the supporting braking figures, such as 6 m at 10 m/s rising to 24 m at 20 m/s
Examiner Tips
- 💡Quote a pair of numbers from the table when you judge a proportionality claim; the figures carry a mark of their own.
- 💡Test proportionality by dividing: if distance ÷ speed is the same for every row, it is proportional.
- 💡Use the words 'increases by a factor of four' rather than 'increases a lot'.
Common Mistakes
- saying both distances double when the speed doubles
- calling the braking distance graph proportional simply because it rises as speed rises
- assuming the driver brakes harder at higher speeds, when the question fixes the braking force
- confusing the braking distance with the time taken to stop