Explain why sound waves are able to travel through both solids and liquids, but S-waves can only travel through solids.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Seismic waves produced by earthquakes include two types: P-waves and S-waves. Scientists use these waves to learn about the structure of the Earth's interior.
Model answer (4 marks)
S-waves are transverse waves, so the particle motion is perpendicular to the direction of propagation.
In a liquid the particles can only move in the direction of the pressure change; a liquid cannot support shear stresses, so it cannot transmit the sideways motion required for a transverse wave.
Sound waves (and P‑waves) are longitudinal waves, with particle motion parallel to the direction of travel.
The back‑and‑forth compression and rarefaction of a longitudinal wave can be transmitted through any medium that can be compressed, so sound waves can travel through solids, liquids and gases.
In a liquid the particles can only move in the direction of the pressure change; a liquid cannot support shear stresses, so it cannot transmit the sideways motion required for a transverse wave.
Sound waves (and P‑waves) are longitudinal waves, with particle motion parallel to the direction of travel.
The back‑and‑forth compression and rarefaction of a longitudinal wave can be transmitted through any medium that can be compressed, so sound waves can travel through solids, liquids and gases.
Examiner tips
- Use the word ‘transverse’ for S‑waves and ‘longitudinal’ for sound waves; link this to the type of particle motion.
- Explain why liquids cannot transmit shear stresses – they cannot support sideways oscillations.
- Show that longitudinal waves rely on compressions/rarefactions, which all media can support.
- Keep the answer concise – 4 short points will earn full marks.
Common mistakes
- Confusing the direction of particle motion – saying S‑waves are longitudinal or sound waves are transverse.
- Claiming liquids can support shear stresses or that S‑waves travel through gases.
- Using vague language such as ‘any medium’ without specifying the role of compressibility.
Mark scheme (4 marks)
- S-waves are transverse waves (oscillations perpendicular to the direction of travel)
- Transverse waves / S-waves cannot travel through liquids because liquids cannot transmit sideways/shear oscillations
- Sound waves (and P-waves) are longitudinal waves (oscillations parallel to the direction of travel)
- Longitudinal waves can travel through liquids because the back-and-forth oscillations (compressions and rarefactions) can be transmitted through any medium that can be compressed
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →