A skateboarder is standing still on a smooth, flat surface. The skateboarder throws a heavy ball horizontally away from them. After releasing the ball, the skateboarder moves in the opposite direction. Explain why the skateboarder moves after releasing the ball, and describe how the speed of the skateboarder compares to the speed of the ball immediately after release.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A skateboarder of mass 60 kg stands stationary on a skateboard on a smooth, flat surface. The skateboarder throws a ball of mass 3 kg horizontally.
Model answer (5 marks)
Before the throw the total momentum of the system is zero.
When the ball is thrown the ball gains momentum in one direction.
Because momentum is conserved, the skateboarder must gain an equal and opposite momentum.
Thus the skateboarder moves in the opposite direction.
Since the skateboarder’s mass (60 kg) is much larger than the ball’s mass (3 kg), the skateboarder’s speed is much smaller than the ball’s speed immediately after the release.
When the ball is thrown the ball gains momentum in one direction.
Because momentum is conserved, the skateboarder must gain an equal and opposite momentum.
Thus the skateboarder moves in the opposite direction.
Since the skateboarder’s mass (60 kg) is much larger than the ball’s mass (3 kg), the skateboarder’s speed is much smaller than the ball’s speed immediately after the release.
Examiner tips
- Show the initial zero‑momentum state first. Use the conservation of momentum statement. Explain the opposite directions. Compare speeds using the mass‑velocity relationship.
- common_mistakes
- :
- Failing to state that the initial momentum is zero. Confusing the direction of motion. Ignoring the mass difference when comparing speeds.
Mark scheme (5 marks)
- Before the throw, the total momentum of the skateboarder and ball system is zero (the system is stationary).
- Momentum is conserved (the total momentum of the system remains zero after the throw).
- The ball gains momentum in one direction, so the skateboarder must gain an equal and opposite momentum, causing the skateboarder to move in the opposite direction.
- The skateboarder moves more slowly than the ball because the skateboarder has a much greater mass than the ball.
- The momentum of the skateboarder equals the momentum of the ball (momentum = mass × velocity), so for a larger mass the velocity must be smaller.
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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