A goalkeeper catches a football that is moving towards them at high speed. The goalkeeper moves their hands backwards as they catch the ball, bringing it to rest over a longer time. Explain, using Newton's laws of motion and the concept of momentum, why moving their hands backwards reduces the force felt by the goalkeeper's hands.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When catching a fast-moving ball, experienced goalkeepers are trained to 'give' with the ball rather than catching it with rigid, stationary hands.
Model answer (5 marks)
The ball has momentum because it has mass and velocity.
When the ball is stopped its momentum changes to zero – a change in momentum.
Moving the hands back increases the time taken for the ball to stop.
By Newton’s second law, force equals the rate of change of momentum.
A longer time means a smaller rate of change of momentum, so the force on the hands is smaller.
When the ball is stopped its momentum changes to zero – a change in momentum.
Moving the hands back increases the time taken for the ball to stop.
By Newton’s second law, force equals the rate of change of momentum.
A longer time means a smaller rate of change of momentum, so the force on the hands is smaller.
Examiner tips
- Use the exact phrase ‘change in momentum’ and ‘rate of change of momentum’
- Show the sequence: momentum → change → time → force
- Keep the answer concise – 5 short points
Common mistakes
- Writing ‘force equals momentum’ instead of ‘force equals rate of change of momentum’
- Omitting the word ‘time’ when explaining the reduction of force
Mark scheme (5 marks)
- The ball has momentum because it has mass and velocity
- When the ball is stopped its momentum changes to zero / there is a change in momentum
- Moving the hands back increases the time taken for the ball to stop
- By Newton's second law / the relationship between force and momentum, force equals the rate of change of momentum
- A longer time means a smaller rate of change of momentum, so the force on the hands is smaller
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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