Explain why the total momentum of a system is conserved when two objects collide.

AQA GCSE Physics (8463) — 4.5.7 Momentum (Phys only) · Explain · 4 marks · View as Markdown

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

Two ice skaters push off from each other whilst standing still on an ice rink. After they push, they move in opposite directions.

Model answer (4 marks)

Before the push both skaters are stationary, so the total momentum of the system is zero.

During the push the skaters exert equal and opposite forces on each other (Newton’s Third Law), so the impulse on each skater is equal and opposite.

After the push the skaters move in opposite directions; their momenta are therefore in opposite directions.

Because the magnitudes of the momenta are equal and the directions opposite, the two momenta cancel, giving a total momentum of zero – the same as before the push.

Examiner tips

  • Use the word ‘equal and opposite’ to show Newton’s Third Law. Show that the initial momentum is zero. Explain that the final momenta cancel to give zero.
  • common_mistakes
  • :
  • Failing to state that the initial momentum is zero. Not recognising that the forces are equal and opposite. Assuming the final momenta are not equal in magnitude.

Mark scheme (4 marks)

  1. Before the event, both skaters are stationary, so the total momentum of the system is zero
  2. The skaters exert equal and opposite forces on each other (Newton's Third Law)
  3. After the push, the skaters move in opposite directions, so their momenta are in opposite directions
  4. The momenta cancel / add to zero, so total momentum after the event is still zero, equalling the total momentum before

Key terms in this question

momentum

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