A rowing boat is stationary on a calm lake. A passenger steps out of the boat onto a jetty. As the passenger moves forwards, the boat moves backwards. Explain why the boat moves backwards, and why the passenger and boat eventually come to rest.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A rowing boat is floating stationary on a calm lake. A passenger of mass 70 kg steps out of the boat onto a jetty. As the passenger moves forwards, the boat moves backwards.
Model answer (4 marks)
The boat and passenger form a closed system. Initially the system is stationary, so the total momentum is zero. When the passenger steps onto the jetty and walks forward, he gains forward momentum. By conservation of momentum the boat must acquire an equal and opposite momentum, so it moves backwards. The force that the passenger exerts on the boat is equal and opposite to the force the boat exerts on the passenger (Newton’s third law), which produces the backward motion of the boat.
Both the passenger and the boat are slowed by resistive forces. The passenger experiences friction with the jetty, and the boat experiences water resistance (drag). These forces act opposite to the directions of motion and gradually reduce the speeds until both the passenger and the boat come to rest.
Both the passenger and the boat are slowed by resistive forces. The passenger experiences friction with the jetty, and the boat experiences water resistance (drag). These forces act opposite to the directions of motion and gradually reduce the speeds until both the passenger and the boat come to rest.
Examiner tips
- Use the term "conservation of momentum" and state that the total momentum before and after is zero.
- Mention Newton’s third law to explain the reaction forces.
- Explain that friction/drag are resistive forces that decelerate both bodies.
Common mistakes
- Failing to state that the total momentum of the system is zero before the passenger steps out.
- Confusing the direction of forces – e.g. saying the boat pushes the passenger forward instead of backward.
- Ignoring the role of friction or water resistance in stopping the motion.
Mark scheme (4 marks)
- The total momentum of the system before the passenger steps out is zero (the system is stationary).
- By conservation of momentum, the total momentum after must also be zero, so the boat moves backwards to give equal and opposite momentum to the passenger moving forwards.
- Newton's third law: the force the passenger exerts on the boat is equal and opposite to the force the boat exerts on the passenger, causing the boat to move in the opposite direction.
- Friction / water resistance (on the boat) and friction (on the passenger from the jetty) act as resistive forces opposing motion, decelerating both the boat and the passenger until they stop.
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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