A cyclist sets off from rest and reaches a speed of 12 m/s over a period of time. The cyclist then travels at a constant speed for several minutes before braking and coming to a stop. Describe the motion of the cyclist during each stage of the journey, and explain what the term 'constant speed' tells us about the forces acting on the cyclist during the middle stage.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A cyclist completes a short journey in three stages: first accelerating from rest, then travelling at a steady speed, and finally braking to a stop.
Model answer (4 marks)
Stage 1: The cyclist starts from rest and accelerates, so the speed increases from 0 m s⁻¹ to 12 m s⁻¹ over the given time.
Stage 2: The cyclist travels at a constant speed of 12 m s⁻¹; the speed does not change during this period.
Stage 3: The cyclist brakes and decelerates, so the speed decreases from 12 m s⁻¹ to 0 m s⁻¹ until the cyclist stops.
The term ‘constant speed’ indicates that the speed is the same at all points in the middle stage. This means that the net horizontal force on the cyclist is zero, so the driving force (pedalling) is exactly balanced by the resistive forces (friction, air resistance).
Stage 2: The cyclist travels at a constant speed of 12 m s⁻¹; the speed does not change during this period.
Stage 3: The cyclist brakes and decelerates, so the speed decreases from 12 m s⁻¹ to 0 m s⁻¹ until the cyclist stops.
The term ‘constant speed’ indicates that the speed is the same at all points in the middle stage. This means that the net horizontal force on the cyclist is zero, so the driving force (pedalling) is exactly balanced by the resistive forces (friction, air resistance).
Examiner tips
- Use the word ‘accelerates’ for stage 1 and ‘decelerates’ for stage 3. Mention the speed value (12 m s⁻¹) for clarity. State that constant speed means the speed is unchanged. Explain that balanced forces give a resultant force of zero.
Common mistakes
- Saying the cyclist ‘moves faster’ instead of ‘accelerates’. Forgetting to mention the speed value 12 m s⁻¹. Claiming that forces are unbalanced during constant speed.”
Mark scheme (4 marks)
- During the first stage, the cyclist accelerates (speed increases from rest / speed increases over time).
- During the final stage, the cyclist decelerates (speed decreases / slows down) until reaching zero.
- Constant speed means the speed does not change / remains the same throughout the middle stage.
- At constant speed, the forces acting on the cyclist are balanced / the resultant force is zero.
Key terms in this question
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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