A cyclist rides along a flat road and then stops pedalling. The cyclist slows down and eventually comes to rest. Explain why the cyclist decelerates after stopping pedalling, referring to the resultant force acting on the cyclist.

Cambridge International IGCSE Physics (0625) — 1.5 Forces · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The cyclist is no longer applying a forward force once pedalling stops. The only forces acting are friction (and air resistance) which oppose the direction of motion. Because there is no forward force to balance these, the resultant force is not zero – it is a net force in the backward direction. A net force opposite to the direction of motion produces a negative acceleration, so the cyclist decelerates until coming to rest.

Examiner tips

  • State that the forward driving force disappears when pedalling stops.
  • Mention friction/air resistance as the opposing forces.
  • Explain that the resultant force is backward, not zero.
  • Link the backward resultant force to negative acceleration (deceleration).

Common mistakes

  • Confusing friction with a forward force; forgetting to mention the absence of a forward force.
  • Saying the cyclist slows because of 'gravity' or 'inertia' instead of friction/air resistance.
  • Using vague terms like 'forces cancel out' without specifying which forces are involved.

Mark scheme (4 marks)

  1. Friction (and/or air resistance) acts on the cyclist opposing the direction of motion
  2. Once pedalling stops, there is no longer a driving/forward force
  3. The resultant force is not zero / there is a net force in the backward direction
  4. A resultant force in the opposite direction to motion causes deceleration / negative acceleration

Key terms in this question

resultant force

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