A cyclist rides along a flat road. The cyclist's legs exert a force to turn the pedals, but the bicycle does not travel as far as it would if all the input energy were converted into kinetic energy. Explain why the bicycle has an efficiency of less than 100% and describe TWO ways in which the efficiency of the bicycle could be increased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist notices that, over time, the chain on their bicycle becomes dry and the wheel bearings feel stiff. They want to improve how efficiently the bicycle transfers energy.
Model answer (4 marks)
The cyclist’s input energy is not all converted into kinetic energy because some of it is lost as waste energy.
1. Friction between the chain, gears and bearings turns the energy into heat and also produces sound, so the useful output is less than the input.
2. Energy can never be created or destroyed (conservation of energy), so the efficiency, defined as useful output divided by input, can never reach 100 %.
Ways to increase efficiency:
3. Apply lubrication to the chain and bearings – this reduces friction and therefore reduces the amount of energy lost as heat.
4. Use thermal insulation or a system that recycles the waste heat (e.g. a heat‑exchanger) so that some of the energy is returned to the system, reducing the overall loss.
1. Friction between the chain, gears and bearings turns the energy into heat and also produces sound, so the useful output is less than the input.
2. Energy can never be created or destroyed (conservation of energy), so the efficiency, defined as useful output divided by input, can never reach 100 %.
Ways to increase efficiency:
3. Apply lubrication to the chain and bearings – this reduces friction and therefore reduces the amount of energy lost as heat.
4. Use thermal insulation or a system that recycles the waste heat (e.g. a heat‑exchanger) so that some of the energy is returned to the system, reducing the overall loss.
Examiner tips
- Use the word "friction" and "heat" to show the source of loss; mention conservation of energy to justify <100 %
- State two specific methods, e.g. lubrication and heat recovery, and explain how each reduces waste energy
Common mistakes
- Failing to mention friction or heat as the waste energy source
- Giving a method that does not actually reduce energy loss, such as changing the rider’s position
Mark scheme (4 marks)
- Some input energy is transferred as waste energy (e.g. heat due to friction between moving parts / sound), rather than as useful kinetic energy
- Because energy can never be created or destroyed (conservation of energy), the useful output energy can never exceed the total input energy, so efficiency is always less than or equal to 1 (100%)
- Applying lubrication (e.g. oiling the chain/bearings) reduces friction between moving parts, reducing the waste energy transfer as heat
- A second valid method of increasing efficiency, e.g. using thermal insulation to prevent heat dissipating to the surroundings / recycling waste energy so it can be reused
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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