A ice skater glides across a smooth ice rink at a constant velocity. She then pushes off a barrier at the edge of the rink. Explain, using Newton's laws, why the skater accelerates away from the barrier after pushing off, and what happens to her velocity once she stops pushing and there is no resultant force acting on her.

OCR GCSE Physics A: Gateway Science (J249) — P2.2 Newton's laws · Explain · 4 marks · View as Markdown

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

An ice skater is practising on a rink. The ice is smooth enough that friction and air resistance can be considered negligible.

Model answer (4 marks)

When she pushes on the barrier, the barrier exerts an equal and opposite force on her (Newton’s Third Law). This reaction force is the only force acting on her, so it is the resultant force and, by Newton’s Second Law, causes her to accelerate away from the barrier.
Once she stops pushing, no external force acts on her. By Newton’s First Law she then continues to move at a constant velocity – the same speed and direction as when she stopped pushing.

Examiner tips

  • Use the correct law names (Third, Second, First) and link each to the situation.
  • Show the chain: push → reaction force → resultant force → acceleration.
  • Mention that after the push the resultant force is zero, so velocity remains constant.

Common mistakes

  • Confusing the direction of the reaction force; remember it is opposite to the push.
  • Forgetting to state that the resultant force is zero after the push.
  • Using vague terms like ‘force’ without specifying which Newton’s law applies.

Mark scheme (4 marks)

  1. When she pushes on the barrier, the barrier exerts an equal and opposite force back on the skater (Newton's Third Law)
  2. This reaction force is the resultant force acting on the skater, causing her to accelerate (Newton's Second Law)
  3. Once she stops pushing there is no resultant force acting on her
  4. She continues to move at a constant velocity (same speed and direction) — Newton's First Law

Key terms in this question

resultant force · constant velocity

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