A student sets up a ripple tank to study water waves. The student notices that a piece of cork floating on the surface bobs up and down as waves pass beneath it, but does not travel across the tank. Explain what this observation tells us about the nature of water waves, and describe the difference between transverse and longitudinal waves.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Water waves in a ripple tank cause a floating cork to oscillate up and down, but the cork stays in roughly the same horizontal position even as waves travel across the surface.
Model answer (5 marks)
The cork moves up and down but does not advance because the water particles vibrate in place while the wave energy propagates – the cork is carried by the oscillation of the water, not by a net transport of water.
In a transverse wave the particle motion is perpendicular to the direction of wave travel. Water waves in a ripple tank are transverse, so the cork’s vertical motion matches the wave’s transverse displacement.
In a longitudinal wave the particle motion is parallel to the direction of wave travel, producing alternating compressions and rarefactions. This is not seen in the ripple tank waves.
In a transverse wave the particle motion is perpendicular to the direction of wave travel. Water waves in a ripple tank are transverse, so the cork’s vertical motion matches the wave’s transverse displacement.
In a longitudinal wave the particle motion is parallel to the direction of wave travel, producing alternating compressions and rarefactions. This is not seen in the ripple tank waves.
Examiner tips
- Show the cork’s vertical motion to prove transverse nature; link to energy transfer without matter transport.
- Use the definition of transverse vs longitudinal motion; mention compressions/rarefactions for longitudinal.
- Keep answer concise – 5 marks, no extra words.
Common mistakes
- Confusing wave type with particle motion; claiming cork moves horizontally.
- Forgetting that ripple tank waves are transverse and that energy, not matter, travels.
- Mixing up transverse and longitudinal definitions or omitting compressions/rarefactions.
Mark scheme (5 marks)
- The cork moving up and down (but not progressing) shows that the water particles oscillate/vibrate but do not travel with the wave / energy is transferred but matter is not
- In a transverse wave, the oscillation/vibration of particles is perpendicular (at right angles) to the direction of wave travel
- Water waves (in the ripple tank) are transverse waves / the up-and-down motion of the cork is consistent with a transverse wave
- In a longitudinal wave, the oscillation/vibration of particles is parallel to (along the same line as) the direction of wave travel
- Longitudinal waves consist of compressions and rarefactions / regions where particles are pushed together (compressions) and spread apart (rarefactions)
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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