A cyclist rides along a flat road and then applies the brakes, coming to a complete stop. Explain the energy transfers that occur from the moment the cyclist is moving at constant speed to the moment they stop, identifying both the useful and waste energy transfers involved.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist travelling at constant speed on a flat road squeezes the brake levers, causing the brake pads to press against the wheel rims. The cyclist slows down and eventually stops.
Model answer (4 marks)
The cyclist’s kinetic energy is stored in the moving system.
When the brakes are applied, friction between the brake pads and wheel rims converts that kinetic energy into thermal energy.
The heat produced in the brakes, wheels and surrounding air is transferred to the environment.
This heat transfer is a waste energy transfer because it does not contribute to the cyclist’s motion.
When the brakes are applied, friction between the brake pads and wheel rims converts that kinetic energy into thermal energy.
The heat produced in the brakes, wheels and surrounding air is transferred to the environment.
This heat transfer is a waste energy transfer because it does not contribute to the cyclist’s motion.
Examiner tips
- Use the term ‘kinetic energy’ for the moving cyclist.
- Mention friction as the mechanism of conversion.
- Identify the heat as the waste energy transfer.
- Keep the answer concise and to the point.
Common mistakes
- Confusing kinetic energy with potential energy.
- Omitting the role of friction.
- Failing to label the heat transfer as waste energy.
Mark scheme (4 marks)
- The moving cyclist has kinetic energy stored in the kinetic energy store
- When the brakes are applied, friction acts between the brake pads and wheel rims
- Kinetic energy is transferred to thermal/heat energy (in the brakes/wheels/surroundings)
- The heat energy transferred to the surroundings is a waste energy transfer because it does not serve the purpose of the system / energy is dissipated
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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