A large lorry and a small car are travelling at the same velocity when they both brake and come to rest. Explain why the lorry has a greater braking distance than the car, even when road conditions and braking force are identical.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Both vehicles are travelling at 20 m/s on a dry, level road. The braking force applied to each vehicle is the same. The lorry has a mass of 12 000 kg and the car has a mass of 1 200 kg.
Model answer (4 marks)
The lorry has a much larger mass than the car.
Because the braking force is the same, Newton’s second law (F = ma) gives a smaller deceleration for the heavier lorry.
A smaller deceleration means the lorry takes longer to reach zero velocity, so it travels a greater distance before stopping.
The lorry also has a larger momentum (p = mv) at the same speed, so the same braking force must change a larger amount of momentum, requiring a longer stopping distance.
Because the braking force is the same, Newton’s second law (F = ma) gives a smaller deceleration for the heavier lorry.
A smaller deceleration means the lorry takes longer to reach zero velocity, so it travels a greater distance before stopping.
The lorry also has a larger momentum (p = mv) at the same speed, so the same braking force must change a larger amount of momentum, requiring a longer stopping distance.
Examiner tips
- Show the mass difference first, then link to F=ma, then to distance via deceleration, finally mention momentum for completeness.
- Use correct units (kg, m/s, N) and clear causal chain.
- Keep answer concise – 4 short sentences is enough for full marks.
Common mistakes
- Confusing force with acceleration – writing ‘force is larger’ instead of ‘deceleration is smaller’.
- Omitting the momentum argument or stating it incorrectly.
- Using informal language or failing to mention the same braking force.”
Mark scheme (4 marks)
- The lorry has a greater mass than the car
- A greater mass produces a smaller acceleration (deceleration) for the same braking force, as force = mass × acceleration / Newton's second law
- Because the lorry decelerates more slowly, it takes longer / travels further before reaching zero velocity
- The lorry has greater momentum (same velocity, larger mass) so a larger change in momentum must occur, requiring more time / distance to stop with the same force
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 →
More Forces, movement, shape and momentum questions
- A rubber bungee cord is stretched by a person jumping from a bridge. At first, t…
- A ball of clay travelling horizontally at 4 m/s collides with an identical stati…
- A driver is travelling at high speed along a dry road when they suddenly notice …
- A rowing boat is stationary on a calm lake. A passenger steps out of the boat on…
- A skateboarder wearing a safety helmet collides with a wall and comes to rest. T…
- A tennis ball travelling at high speed is caught by a fielder. The fielder moves…