A stationary ice skater catches a ball thrown towards her. After catching the ball, the skater and ball move together. Explain what happens to the momentum of the skater and ball during this event.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A stationary ice skater stands on a frictionless ice rink. A ball of mass 0.5 kg is thrown horizontally towards her. She catches the ball and moves backwards.
Model answer (4 marks)
1. Before the catch the skater has zero momentum because she is stationary.
2. The ball has momentum (p_{ ext{ball}}=m_{ ext{ball}}v_{ ext{ball}}) in the direction it was thrown.
3. Momentum is conserved, so the total momentum before the catch equals the total momentum after the catch.
4. After the catch the ball’s momentum decreases to zero relative to the skater, and the skater gains an equal amount of momentum in the same direction, causing her to move backwards.
2. The ball has momentum (p_{ ext{ball}}=m_{ ext{ball}}v_{ ext{ball}}) in the direction it was thrown.
3. Momentum is conserved, so the total momentum before the catch equals the total momentum after the catch.
4. After the catch the ball’s momentum decreases to zero relative to the skater, and the skater gains an equal amount of momentum in the same direction, causing her to move backwards.
Examiner tips
- Use the conservation of momentum explicitly; mention total momentum before = after.
- Show the ball’s momentum before and after; note it reduces to zero relative to the skater.
- Explain that the skater’s momentum increases in the ball’s original direction.
- Keep the answer concise and use correct terminology (momentum, conservation).
Common mistakes
- Failing to state that the skater’s initial momentum is zero.
- Confusing the direction of the skater’s motion with the ball’s direction.
- Not recognising that the ball’s momentum relative to the skater becomes zero after the catch.
Mark scheme (4 marks)
- Before the event, the skater has zero momentum (because she is stationary)
- The total momentum before the event equals the total momentum after the event (conservation of momentum)
- The ball has momentum before the catch / the ball's momentum decreases after being caught
- The skater gains momentum (in the same direction as the ball was travelling), so the skater moves in the direction the ball was thrown
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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