A cyclist rides along a flat road and then applies the brakes to come to a stop. Explain what happens to the kinetic energy of the cyclist and bicycle during braking, and explain one method the cyclist could use before the journey to reduce the amount of energy wasted when cycling at a constant speed.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist and their bicycle have a combined mass of 80 kg and are travelling at 10 m/s before the brakes are applied.
Model answer (5 marks)
The cyclist and bicycle possess kinetic energy because they are moving. When the brakes are applied, a friction force acts between the brake pads and the wheel. This friction force does negative work on the system, converting the kinetic energy into heat or thermal energy in the brakes and the surrounding air. Thus the kinetic energy is lost as heat.
Before a ride the cyclist could lubricate the moving parts of the bicycle, such as the chain and wheel bearings. Lubrication reduces the friction between these parts, so less kinetic energy is wasted as heat while cycling at a constant speed.
Before a ride the cyclist could lubricate the moving parts of the bicycle, such as the chain and wheel bearings. Lubrication reduces the friction between these parts, so less kinetic energy is wasted as heat while cycling at a constant speed.
Examiner tips
- Use the word ‘friction’ and ‘heat’ – the mark scheme rewards explicit mention of the energy conversion.
- Show the chain of reasoning: kinetic energy → friction → heat.
- Mention lubrication as a specific method and explain its effect on friction.
Common mistakes
- Failing to mention that the kinetic energy is converted to heat rather than simply ‘lost’.
- Using vague terms like ‘energy wasted’ without linking to friction.
- Omitting the specific example of lubricating the chain or bearings.
Mark scheme (5 marks)
- The cyclist and bicycle have kinetic energy because they are moving
- When the brakes are applied, a friction force acts between the brake pads and the wheel
- The kinetic energy is transferred to heat / thermal energy (in the brakes and surroundings)
- The cyclist could lubricate the moving parts (e.g. chain / wheel bearings) of the bicycle
- Lubrication reduces friction between moving parts, so less energy is wasted as heat during cycling
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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