A cyclist is riding along a flat road and begins to slow down when she stops pedalling. Explain why the cyclist decelerates and describe what eventually happens to her motion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist stops pedalling whilst travelling along a flat, straight road. She does not apply the brakes.
Model answer (4 marks)
1. Friction between the tyres and the road, air resistance and drag act on the cyclist in the direction opposite to her motion.
2. When she stops pedalling there is no longer a driving or thrust force to balance these resistive forces.
3. The unbalanced resistive forces produce a resultant force backwards, which causes her to decelerate.
4. As the deceleration continues, her speed decreases until she eventually comes to rest, because the resistive forces bring her to a halt.
2. When she stops pedalling there is no longer a driving or thrust force to balance these resistive forces.
3. The unbalanced resistive forces produce a resultant force backwards, which causes her to decelerate.
4. As the deceleration continues, her speed decreases until she eventually comes to rest, because the resistive forces bring her to a halt.
Examiner tips
- Use the term ‘resistive forces’ (friction, air resistance, drag).
- Explain that the absence of a forward thrust leaves a net backward force.
- Show the chain: no thrust → net backward force → deceleration → stop.
- Mention that the motion is along a flat road, so no gravitational component changes the analysis.
Common mistakes
- Forgetting to mention air resistance or drag as resistive forces.
- Saying the cyclist ‘slows down because she stops pedalling’ without linking to the resultant force.
- Claiming the cyclist stops instantly or ignoring that deceleration is gradual.
Mark scheme (4 marks)
- Friction / air resistance / drag acts on the cyclist (in the opposite direction to motion)
- When she stops pedalling there is no longer a driving / thrust force
- There is a resultant force acting backwards / in the opposite direction to motion, causing deceleration
- The cyclist eventually comes to rest / stops (because the resistive forces bring her to a halt)
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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