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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A lorry drives past a stationary observer on the pavement. The observer hears the engine noise clearly as the lorry passes.
Model answer (5 marks)
Sound is a longitudinal wave, so the oscillations of the air particles are parallel to the direction of travel. The lorry’s engine creates alternating compressions and rarefactions that propagate through the air. The air particles themselves only vibrate about their equilibrium positions; they do not move permanently towards the observer. The disturbance carries energy, not the particles, so the energy is transferred through the air. The wave reaches the observer’s eardrum, causing it to vibrate and producing the continuous sound heard.
Examiner tips
- Use the term ‘longitudinal wave’ and ‘compressions/rarefactions’.
- Explain that particles oscillate about equilibrium, not move with the wave.
- Show that energy, not matter, is transmitted.
- Mention the eardrum’s vibration as the final step.
Mark scheme (5 marks)
- Sound is a longitudinal wave / the oscillations are parallel to the direction of travel / energy transfer
- The wave travels as compressions and rarefactions in the air
- The air particles oscillate / vibrate about their equilibrium / undisturbed position
- It is energy / the disturbance that is transferred through the air, not the air particles themselves
- 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
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Progressive and Stationary Waves questions
- Explain how stationary waves are formed in a stretched string fixed at both ends…
- A doctor uses ultrasound to scan a patient's liver. The ultrasound waves travel …
- A student sets up a stretched string fixed at both ends and plucks it to produce…
- A fishing boat uses sonar to locate a shoal of fish beneath it. A pulse of ultra…
- A student claps two wooden boards together once while standing 85 m from a large…
- Explain how longitudinal waves travel through a medium, referring to compression…