# Explain why a stationary observer standing near a road hears a continuous sound from a passing lorry as a wave, yet the air through which the sound travels does not permanently move towards the observer.

> AQA A-Level Physics (7408) — 3.3.1 Progressive and Stationary Waves · Explain · 5 marks

> A lorry drives past a stationary observer on the pavement. The observer hears the engine noise clearly as the lorry passes.

## Mark scheme (5 marks)

1. Sound is a longitudinal wave / the oscillations are parallel to the direction of travel / energy transfer
2. The wave travels as compressions and rarefactions in the air
3. The air particles oscillate / vibrate about their equilibrium / undisturbed position
4. It is energy / the disturbance that is transferred through the air, not the air particles themselves
5. The wave travels from the lorry (source) to the observer, transferring energy so the observer's eardrum receives the vibrations / is caused to vibrate

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
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