A cyclist rides along a flat road and then brakes to a complete stop. Explain what happens to the kinetic energy of the cyclist and bicycle as they slow down, and explain why the total energy of the cyclist-bicycle system is still conserved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist travelling at speed applies the brakes and comes to a complete stop on a flat road.
Model answer (4 marks)
The kinetic energy of the cyclist and bicycle decreases as the speed falls to zero.
The lost kinetic energy is transferred to heat by the brakes – the friction between the brake pads and the wheels converts the kinetic energy into thermal energy.
This heat is then dissipated to the surroundings, for example into the air and the road.
Energy is still conserved because the total energy of the system has not been created or destroyed; it has simply changed form from kinetic to thermal and then spread into the environment.
The lost kinetic energy is transferred to heat by the brakes – the friction between the brake pads and the wheels converts the kinetic energy into thermal energy.
This heat is then dissipated to the surroundings, for example into the air and the road.
Energy is still conserved because the total energy of the system has not been created or destroyed; it has simply changed form from kinetic to thermal and then spread into the environment.
Examiner tips
- Use the word ‘decreases’ to show the change in kinetic energy. Mention the specific mechanism (friction in the brakes) for the energy transfer. Explain that the heat is lost to the surroundings, not destroyed. Show that the total energy remains constant by linking the transfer to conservation.
Common mistakes
- Saying the kinetic energy ‘increases’ or ‘stays the same’. Forgetting to mention the heat transfer via friction. Claiming that energy is ‘destroyed’ rather than ‘transferred’.
Mark scheme (4 marks)
- The kinetic energy of the cyclist and bicycle decreases (as they slow down / as speed decreases).
- The kinetic energy is transferred to heat (thermal energy) via the brakes / through friction between the brake pads and wheels.
- This heat energy is dissipated to the surroundings (e.g. the air / road).
- Energy is conserved because it has been transferred / stored in a different form rather than created or destroyed — the total energy remains constant.
Key terms in this question
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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