A student claps their hands once whilst standing a large distance from a tall cliff face. They hear an echo a short time later. Explain how the student could use this situation to determine the speed of sound in air, and state why the distance used in the calculation must be twice the distance between the student and the cliff.

Pearson Edexcel International GCSE Physics (4PH1) — 3.3 Light and sound · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Measure the distance, d, from the student to the cliff face.
Measure the time, t, between the clap and the echo.
Use v = 2d / t to calculate the speed of sound.
The sound travels to the cliff and back, so the total distance is twice the one‑way distance.

Examiner tips

  • State the distance and time measurements first. Show the formula v = 2d/t and explain the factor 2. Use correct units (m, s, m/s).
  • Use the word "echo" to show you understand the round‑trip nature of the sound.

Common mistakes

  • Using v = d/t instead of v = 2d/t. Forgetting to double the distance. Not specifying that the measured time is the round‑trip time.

Mark scheme (4 marks)

  1. Measure the distance between the student and the cliff face
  2. Measure the time between the clap and the echo being heard
  3. Use speed = distance / time to calculate the speed of sound
  4. The sound must travel to the cliff and back, so the total distance travelled by the sound is twice the distance between the student and the cliff

Key terms in this question

echo · speed of sound

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