A skateboarder standing still on a smooth, flat surface throws a heavy ball horizontally away from them. After throwing the ball, the skateboarder moves in the opposite direction to the ball. Using the concept of conservation of momentum, explain why the skateboarder moves after releasing the ball, and describe what determines how fast the skateboarder moves.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Before the throw, the skateboarder and ball are both stationary. The skateboarder has a mass of 60 kg. The ball is thrown horizontally.
Model answer (5 marks)
1. The system (skateboarder + ball) is initially at rest, so its total momentum is zero.
2. Momentum is conserved; after the throw the total momentum must still be zero.
3. The ball acquires momentum in the direction it is thrown.
4. To keep the total zero, the skateboarder must acquire an equal magnitude of momentum in the opposite direction.
5. The skateboarder's speed is given by (v_s= rac{m_b,v_b}{m_s}); it increases with the ball’s mass or speed and decreases with the skateboarder’s mass.
2. Momentum is conserved; after the throw the total momentum must still be zero.
3. The ball acquires momentum in the direction it is thrown.
4. To keep the total zero, the skateboarder must acquire an equal magnitude of momentum in the opposite direction.
5. The skateboarder's speed is given by (v_s= rac{m_b,v_b}{m_s}); it increases with the ball’s mass or speed and decreases with the skateboarder’s mass.
Examiner tips
- Use the conservation of momentum equation explicitly; show the zero‑momentum before and after. Mention the opposite directions of the two momenta. State the formula for the skateboarder’s speed and explain the dependence on masses and ball speed.
Common mistakes
- Assuming the skateboarder’s speed is independent of the ball’s mass or speed. Forgetting that the total momentum must remain zero. Using the wrong sign for the skateboarder’s momentum (same direction as the ball).
Mark scheme (5 marks)
- The total momentum of the skateboarder and ball before the throw is zero (system is stationary).
- By conservation of momentum, the total momentum after the throw must also equal zero.
- The ball gains momentum in one direction, so the skateboarder must gain an equal and opposite momentum to keep the total at zero.
- The speed of the skateboarder depends on the mass and speed of the thrown ball — a heavier or faster-thrown ball causes the skateboarder to move faster.
- The speed of the skateboarder is also affected by the skateboarder's own mass — a greater mass of skateboarder means a lower speed for the same ball momentum.
Key terms in this question
momentum · conservation of momentum
Related
- All Eduqas A-Level Physics 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 firefighter stands on a stationary boat floating on a calm lake. The firefight…
- A child standing on a stationary ice rink pushes a heavy sledge horizontally awa…
- A railway wagon of mass 4000 kg is moving at 3 m/s along a level track when it c…