A bungee jumper leaps from a bridge. At the moment of jumping, the jumper accelerates downwards rapidly. As the bungee cord stretches, the jumper eventually reaches a point where they move at a constant velocity before the cord pulls them back upwards. Explain why the jumper first accelerates and then reaches a constant velocity during the downward descent.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A bungee cord is an elastic cord that stretches when a force is applied to it. As the jumper falls, the cord begins to stretch and exerts an increasing upward force on the jumper.
Model answer (4 marks)
1. At the moment of the jump the only significant downward force is the jumper’s weight (mg). 2. With no opposing force the resultant force is downwards, so by Newton’s Second Law the jumper accelerates downwards. 3. As the cord stretches, its elastic (tension) force increases. 4. When the upward tension equals the downward weight the resultant force is zero, so the jumper’s speed becomes constant.
Examiner tips
- Use the word ‘accelerate’ for the first part and ‘constant velocity’ for the second.
- Show the forces (weight and tension) and state that they balance at constant speed.
- Mention Newton’s Second Law for the acceleration stage.
- Keep the answer concise – 4 short points are enough for full marks.
Common mistakes
- Confusing the direction of the tension force (writing it as downward).
- Forgetting to state that the resultant force is zero at constant velocity.
- Using vague terms like ‘gravity’ instead of ‘weight’ or ‘tension’.”
Mark scheme (4 marks)
- At the moment of jumping, the only significant downward force is the jumper's weight / gravitational force
- The resultant force is downwards so the jumper accelerates downwards (Newton's First / Second Law)
- As the cord stretches, the upward (elastic / tension) force from the cord increases
- Constant velocity is reached when the upward force from the cord equals the downward weight, giving a resultant force of zero / balanced forces
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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