# A pendulum clock keeps accurate time because its pendulum swings in simple harmonic motion. Explain what is meant by simple harmonic motion and describe how the amplitude and time period of the pendulum's swing change as the clock continues to run over several hours without any external energy input.

> Eduqas A-Level Physics — 5.1 Simple harmonic motion · Explain · 5 marks

> A grandfather clock uses a long pendulum that swings back and forth to regulate the movement of its hands. Over time, air resistance and friction at the pivot gradually remove energy from the pendulum.

## Mark scheme (5 marks)

1. Simple harmonic motion involves a restoring force (or acceleration) that acts towards the equilibrium position
2. The restoring force (or acceleration) is proportional to the displacement from the equilibrium position
3. The amplitude decreases over time as energy is removed by air resistance / friction
4. The time period remains constant / does not change as amplitude decreases
5. This is why the clock keeps accurate time — the constant time period means each swing still marks the same interval of time

## Key terms

- [simple harmonic motion](https://www.gradenine.co.uk/glossary/simple-harmonic-motion)
- [amplitude](https://www.gradenine.co.uk/glossary/amplitude)
- [time period](https://www.gradenine.co.uk/glossary/time-period)

## Related

- [Revision notes for Eduqas A-Level Physics](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-pendulum-clock-keeps-accurate-time-a613d62a) · Published by Druglandscape Ltd.