A cyclist rides along a flat road at a constant velocity. Explain why the cyclist must continue to pedal in order to maintain this constant velocity, and describe what would happen to the cyclist's motion if they stopped pedalling.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist is travelling along a flat, straight road at a constant velocity of 12 m/s. The cyclist continues to pedal to maintain this speed.
Model answer (5 marks)
At constant velocity the net force on the cyclist is zero – the forces are balanced.
The cyclist is subject to resistive forces such as friction between the tyres and the road and air resistance. These forces act opposite to the direction of motion.
The cyclist pedals to produce a thrust (driving force) that is equal in magnitude and opposite in direction to the resistive forces, so the resultant force remains zero and the speed stays constant.
If the cyclist stops pedalling the thrust disappears. The resistive forces are no longer balanced, so a resultant force now acts in the direction of the resistive forces. The cyclist therefore experiences a net force opposite to the motion and decelerates, eventually coming to rest.
The cyclist is subject to resistive forces such as friction between the tyres and the road and air resistance. These forces act opposite to the direction of motion.
The cyclist pedals to produce a thrust (driving force) that is equal in magnitude and opposite in direction to the resistive forces, so the resultant force remains zero and the speed stays constant.
If the cyclist stops pedalling the thrust disappears. The resistive forces are no longer balanced, so a resultant force now acts in the direction of the resistive forces. The cyclist therefore experiences a net force opposite to the motion and decelerates, eventually coming to rest.
Examiner tips
- Use the phrase "net force is zero" to show balance of forces.
- Mention both friction and air resistance as the resistive forces.
- Explain that pedalling provides the thrust that balances these forces.
- State that stopping pedalling removes the thrust, giving a non‑zero resultant force opposite to motion.
Common mistakes
- Failing to identify the resistive forces (e.g. only mentioning friction and ignoring air resistance).
- Confusing the direction of the resultant force – saying it acts forward instead of opposite to motion.
- Not linking the removal of thrust to the appearance of a non‑zero resultant force and subsequent deceleration.
Mark scheme (5 marks)
- At constant velocity, the resultant force is zero (forces are balanced)
- Friction and/or air resistance act on the cyclist in the opposite direction to motion
- The pedalling provides a thrust / driving force to balance (equal) these resistive forces
- If the cyclist stops pedalling, the thrust is removed so the resultant force is no longer zero / a resultant force now acts in the direction of friction/air resistance
- The cyclist decelerates (slows down) because the resultant force opposes motion
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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