A cyclist travels along a straight, flat road. She pedals hard for a while and then stops pedalling, but does not brake. Explain what happens to the motion of the cyclist after she stops pedalling, until she eventually comes to rest.

WJEC GCSE Physics (Wales) — 2.3 Forces and motion · 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 driving force from pedalling is no longer acting.
Friction (and/or air resistance) continues to act on the cyclist, opposing her motion.
The resultant force is no longer zero – the resistive forces are unbalanced, so she decelerates.
She eventually comes to rest because the resistive forces remove all her kinetic energy over a distance until her speed reaches zero.

Examiner tips

  • State the loss of the driving force first. Mention the resistive forces and that they oppose motion. Explain that the net force is now unbalanced, leading to deceleration. Show that kinetic energy is dissipated until speed is zero.

Common mistakes

  • Forgetting to mention friction or air resistance. Saying the cyclist keeps moving without braking. Confusing acceleration with deceleration.

Mark scheme (4 marks)

  1. The driving force from pedalling is no longer acting (once she stops pedalling)
  2. Friction (and/or air resistance) continues to act on the cyclist, opposing her motion
  3. The resultant force is no longer zero / forces are unbalanced, so she decelerates (slows down)
  4. She eventually comes to rest because the resistive forces have removed all her kinetic energy / acted over a distance until her speed reaches zero

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