A physics teacher places a ping-pong ball on the surface of water in a ripple tank. The teacher then switches on the oscillating paddle to produce water waves. Explain why the ping-pong ball does not travel across the tank in the direction the waves are moving, and describe what the ping-pong ball does instead.

OCR A-Level Physics A (H556) — 5.1 Thermal physics · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A ripple tank is a shallow glass tank of water. An oscillating paddle produces regular water waves that travel across the surface. A ping-pong ball is floating on the water's surface when the waves are switched on.

Model answer (5 marks)

The ping‑pong ball is a particle of the medium, not the wave itself. In a ripple tank the waves are transverse: the water particles (and the ball) oscillate vertically, perpendicular to the direction of wave propagation. Therefore the ball does not move horizontally with the wave. Instead, as a crest passes it is lifted upward, then it falls with the trough, oscillating up and down with a displacement equal to the wave amplitude. This shows that energy is transferred by the wave while the medium (water and ball) only undergoes local motion.

Examiner tips

  • Use the word ‘transverse’ and emphasise vertical particle motion. Mention that the ball’s horizontal position remains essentially unchanged. Show the displacement equals the amplitude. Link to the idea that waves transfer energy without moving matter.

Common mistakes

  • Confusing the wave with the medium and saying the ball moves horizontally. Forgetting to state that the motion is vertical. Not linking the motion to the wave’s amplitude or energy transfer.

Mark scheme (5 marks)

  1. The ping-pong ball (particle) does not travel in the direction of wave travel because it is the wave/energy that travels, not the medium/water itself.
  2. Water waves are transverse waves, meaning particle oscillations are perpendicular to the direction of energy transfer.
  3. The ping-pong ball moves up and down (vertically) as the wave passes.
  4. The ball oscillates between crests and troughs — moving up to the crest and down to the trough — with a displacement equal to the amplitude of the wave.
  5. This provides evidence that a wave transfers energy without transferring matter/the medium.

Key terms in this question

wave

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