A student claps their hands once whilst standing a large distance from a tall cliff face. They hear two sounds: the original clap and then an echo a short time later. Explain how the echo is produced and state what measurements the student would need to take in order to determine the speed of sound in air.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sound waves are longitudinal waves that can be reflected. When a sound wave strikes a solid boundary, it bounces back towards the source. A student uses this property to measure the speed of sound.
Model answer (4 marks)
The clap produces a sound wave that propagates outward. When it reaches the cliff face the wave is reflected back as a longitudinal wave. The reflected wave travels back to the student and is heard as an echo.
To find the speed of sound the student must:
1. Measure the distance, d, from their position to the cliff face.
2. Measure the time, Δt, between hearing the original clap and the echo.
3. Calculate the speed using v = 2d / Δt (the factor 2 accounts for the round‑trip distance).
To find the speed of sound the student must:
1. Measure the distance, d, from their position to the cliff face.
2. Measure the time, Δt, between hearing the original clap and the echo.
3. Calculate the speed using v = 2d / Δt (the factor 2 accounts for the round‑trip distance).
Examiner tips
- Use the word ‘reflected’ to show understanding of wave behaviour. State the two measurements explicitly. Show the formula v = 2d/Δt and explain the factor 2. Keep the answer concise and to the point.
Common mistakes
- Confusing the one‑way distance with the total distance travelled. Forgetting to double the distance in the speed calculation. Using the wrong unit for distance or time, or omitting units in the final answer.
Mark scheme (4 marks)
- The sound wave from the clap travels to the cliff face and is reflected back towards the student.
- The reflected wave reaching the student's ears is heard as the echo.
- The student must measure the distance between themselves and the cliff face.
- The student must measure the time between the original clap and the echo being heard, and use speed = distance / time where the total distance is twice the distance to the cliff.
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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