A child on a playground swing moves back and forth in simple harmonic motion. Describe the changes in speed and the direction of the restoring force as the child moves from the maximum displacement on one side to the maximum displacement on the other side.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A child is given a single push on a playground swing. The swing then moves freely, reaching a maximum displacement on each side before returning towards the centre.
Model answer (5 marks)
At the maximum displacement the child is momentarily stationary – the speed is zero.
As the child moves towards the centre, the restoring force pulls them back, so they accelerate and their speed increases.
They reach maximum speed at the equilibrium (centre) position.
From the centre to the opposite maximum displacement the restoring force still pulls towards the centre, so the child decelerates and the speed decreases.
The restoring force always acts towards the equilibrium position, opposing the displacement.
As the child moves towards the centre, the restoring force pulls them back, so they accelerate and their speed increases.
They reach maximum speed at the equilibrium (centre) position.
From the centre to the opposite maximum displacement the restoring force still pulls towards the centre, so the child decelerates and the speed decreases.
The restoring force always acts towards the equilibrium position, opposing the displacement.
Examiner tips
- Use the word ‘momentarily’ for zero speed at extremes. Mention that speed is maximum at the centre. State that the restoring force always points towards the centre, not away.
- common_mistakes
- :
- Forgetting that speed is zero at the maximum displacement. Saying the restoring force points away from the centre. Not recognising that speed is greatest at the equilibrium position.
Mark scheme (5 marks)
- At maximum displacement the child is momentarily stationary / speed is zero
- The child accelerates (speeds up) as they move towards the equilibrium / centre position
- The child reaches maximum speed at the equilibrium / centre position
- The child then decelerates (slows down) moving from the centre to the opposite maximum displacement
- The restoring force always acts towards the equilibrium / centre position (opposing the displacement)
Key terms in this question
simple harmonic motion · maximum displacement · restoring force
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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