A person stands on a stationary rowing boat that is floating on a calm lake. The person jumps horizontally off the front of the boat onto a jetty. Explain what happens to the boat when the person jumps, using the principle of conservation of momentum.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The lake surface is smooth enough that friction between the boat and the water can be ignored. Before the jump, both the person and the boat are completely stationary.
Model answer (5 marks)
Before the jump the total momentum of the system (person + boat) is zero. After the jump the total momentum must still be zero (conservation of momentum). The person gains forward momentum toward the jetty. Therefore the boat must gain an equal and opposite momentum, moving backwards away from the jetty. The magnitude of the boat’s momentum equals that of the person’s, keeping the total momentum zero.
Examiner tips
- State the initial zero momentum of the system
- Explain that momentum is conserved after the jump
- Show the direction of the person’s momentum and the opposite direction for the boat
- Mention that the magnitudes are equal
Common mistakes
- Ignoring the initial zero momentum of the system
- Failing to specify the direction of the boat’s motion
- Assuming the boat’s momentum is different in magnitude from the person’s
Mark scheme (5 marks)
- Before the jump the total momentum of the system (person + boat) is zero
- The total momentum of the system must remain zero after the jump (conservation of momentum)
- The person gains momentum in the forward direction (towards the jetty)
- The boat must gain an equal and opposite momentum (moves away from the jetty / backwards)
- The boat's momentum is equal in magnitude to the person's momentum so that the total remains zero
Key terms in this question
momentum · conservation of momentum
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Momentum (A-Level only) questions
- A student pushes a stationary trolley along a smooth, flat surface. After the pu…
- A railway carriage of mass 4000 kg is moving at 3 m/s along a level track. It co…
- A firefighter standing on a smooth, frictionless platform fires a water hose hor…
- A skateboarder is standing still on a smooth, flat surface. The skateboarder thr…