A car travelling at high speed brakes suddenly and comes to a stop. Explain why a driver who is tired takes a longer stopping distance than a fully alert driver travelling at the same speed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Two drivers are travelling at the same speed along a straight, dry road in identical cars. Driver A is fully alert, whilst Driver B has not slept for 24 hours. Both drivers apply the brakes when they see the same hazard ahead.
Model answer (4 marks)
A tired driver has a longer reaction time, so the car travels further before the brakes are applied – this is the thinking distance. The braking distance, which depends only on speed, road and car conditions, is the same for both drivers. Stopping distance is the sum of thinking and braking distances, so the greater thinking distance gives the tired driver a longer overall stopping distance.
Examiner tips
- Mention reaction time first, then thinking distance, then note braking distance is unchanged, finally sum to give stopping distance
- Use exact terms: reaction time, thinking distance, braking distance, stopping distance
Common mistakes
- Confusing reaction time with braking distance; writing that braking distance is longer for the tired driver
- Omitting the fact that braking distance is identical for both drivers
Mark scheme (4 marks)
- Tiredness increases the driver's reaction time
- A longer reaction time means the vehicle travels further before the brakes are applied, increasing the thinking distance
- The braking distance is the same for both drivers (road and car conditions are identical and speed is the same)
- Stopping distance is the sum of thinking distance and braking distance, so the greater thinking distance means a longer overall stopping distance for the tired driver
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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