A cyclist rides along a flat road and then applies the brakes to stop. Explain what happens to the energy stored in the system as the cyclist slows down, and describe one way the cyclist could reduce the amount of energy wasted during braking.

AQA GCSE Physics (8463) — 4.1.2 Conservation and dissipation of energy · 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 and bicycle possess kinetic energy while moving.
When the brakes are applied, that kinetic energy is converted into thermal energy in the brake pads and surrounding air.
The thermal energy is dissipated as waste heat and cannot be reused.
If the cyclist uses a lubricated brake system (e.g. oil on the brake mechanism), friction is reduced, so less kinetic energy is lost as heat.

Examiner tips

  • Use the term "kinetic energy" for the moving system. Explain the conversion to "thermal energy" in the brakes. Mention that this energy is "wasted". Show the reduction of friction by lubrication as a way to save energy.

Common mistakes

  • Confusing kinetic energy with potential energy. Saying the energy is "used up" instead of "converted to heat". Omitting the word "thermal" or "heat" when describing the energy loss.

Mark scheme (4 marks)

  1. The cyclist (and bicycle) have kinetic energy store whilst moving
  2. When the brakes are applied, kinetic energy is transferred to the thermal energy store (of the brakes/surroundings)
  3. This thermal energy is dissipated / is waste energy (and cannot easily be used again)
  4. Lubrication (e.g. oil on brake mechanism) reduces friction, so less energy is wasted as heat

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