A cyclist rides along a flat road at a constant speed. Explain why the cyclist must keep pedalling to maintain this constant speed, and describe what happens to the energy that is not used to keep the cyclist moving forward.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist travels along a flat road at a constant speed. Despite the road being flat and the speed not changing, the cyclist must continue to apply a force by pedalling.
Model answer (4 marks)
The cyclist must keep pedalling because a resistive force – for example friction in the chain, tyre‑road friction and air resistance – opposes his motion. To keep the speed constant the driving force from the pedals must equal this resistive force, so work is continually done to overcome it.
The energy that is not used to move the cyclist forward is dissipated as heat. It is transferred to the surroundings – the rider’s body, the tyres and the air – raising their temperature.
The energy that is not used to move the cyclist forward is dissipated as heat. It is transferred to the surroundings – the rider’s body, the tyres and the air – raising their temperature.
Examiner tips
- Show the balance of forces (driving = resistive) to justify constant speed
- Explain that work done against resistance becomes heat
- Use terms: resistive force, work, energy dissipation, thermal energy
Common mistakes
- Failing to mention the resistive force or the need for equal forces
- Confusing kinetic energy change with energy dissipated
- Not specifying that the heat is transferred to the surroundings
Mark scheme (4 marks)
- There is a resistive force (such as friction / air resistance) acting against the cyclist's motion
- The cyclist must do work to overcome this resistive force / the driving force must equal the resistive force to maintain constant speed
- Energy is dissipated / transferred to the surroundings
- The dissipated energy is transferred as heat (thermal energy), causing a rise in temperature of the surroundings / the cyclist's body / the tyres
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Energy — forces doing work questions
- A delivery driver pushes a heavy box along a warehouse floor at a constant speed…
- A skier starts from rest at the top of a snow-covered slope and slides to the bo…
- A tennis ball is dropped from rest and falls vertically downwards onto a hard fl…
- A worker uses a machine to lift heavy crates from the ground onto a shelf. The m…