A skateboarder stands still on a skateboard on a smooth, flat surface. The skateboarder throws a heavy bag horizontally. After throwing the bag, the skateboarder and skateboard move in the opposite direction. Explain why the skateboarder moves after throwing the bag, and describe what happens to the speed of the skateboarder compared to the speed of the bag.

Edexcel A-Level Physics (9PH0) — 2.2 Forces, Newton's laws and momentum · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A skateboarder of mass 60 kg stands on a skateboard on a smooth, flat surface. The skateboarder throws a heavy bag of mass 10 kg horizontally. Before throwing the bag, both the skateboarder and the bag are stationary.

Model answer (5 marks)

Before the throw the total momentum of the system is zero. Momentum is conserved, so after the throw the total momentum must still be zero. The bag therefore gains momentum in one direction, and the skateboarder must gain an equal amount of momentum in the opposite direction. Because the skateboarder’s mass (60 kg) is greater than the bag’s mass (10 kg), the skateboarder’s velocity is smaller. Thus the skateboarder moves more slowly than the bag.

Examiner tips

  • Use the conservation of momentum explicitly; state total momentum before and after.
  • Show that the bag’s momentum is in one direction and the skateboarder’s is opposite.
  • Explain the mass difference to justify the relative speeds.
  • Use the equation p=mv to link mass and velocity.

Common mistakes

  • Failing to state that total momentum is zero before the throw.
  • Confusing the direction of the skateboarder’s motion with the bag’s.
  • Not linking the greater mass to the lower speed.

Mark scheme (5 marks)

  1. Before the throw, the total momentum of the system is zero (system is stationary)
  2. The total momentum must remain zero after the throw because momentum is conserved
  3. The bag gains momentum in one direction, so the skateboarder must gain equal momentum in the opposite direction
  4. The skateboarder has a greater mass than the bag
  5. Because momentum = mass × velocity, the skateboarder moves more slowly than the bag

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