A student claps two wooden boards together once while standing 85 m from a large flat wall. She hears an echo shortly after the original clap. Describe how the student could use this observation to determine the speed of sound in air, and explain why the wave she hears as an echo is the same type of wave as the original sound wave.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Measure the time between the original clap and the echo being heard.
2. The sound travels to the wall and back, so the total distance is 2 × 85 m = 170 m.
3. Use the wave‑speed equation: speed = distance ÷ time. Insert the measured time to calculate the speed of sound.
4. The original sound is a longitudinal wave, with particle oscillations parallel to the direction of propagation.
5. The echo is produced by reflection of the same longitudinal wave from the wall; the wave type is unchanged, so the echo is also a longitudinal wave.
2. The sound travels to the wall and back, so the total distance is 2 × 85 m = 170 m.
3. Use the wave‑speed equation: speed = distance ÷ time. Insert the measured time to calculate the speed of sound.
4. The original sound is a longitudinal wave, with particle oscillations parallel to the direction of propagation.
5. The echo is produced by reflection of the same longitudinal wave from the wall; the wave type is unchanged, so the echo is also a longitudinal wave.
Examiner tips
- Use a stopwatch or timer to record the time accurately; include the unit (s).
- Show the calculation step: speed = 170 m ÷ t, and give the result in m s⁻¹.
- Mention that the wave remains longitudinal after reflection, not a transverse wave.
- Use the exact terms "longitudinal wave" and "reflection" as the mark scheme expects.
Common mistakes
- Failing to double the distance (using 85 m instead of 170 m).
- Not stating that the echo is still a longitudinal wave, or confusing it with a transverse wave.
- Omitting the unit for speed or the time measurement.
Mark scheme (5 marks)
- Measure the time between the original clap and the echo being heard
- Total distance travelled by the sound is 2 × 85 m = 170 m (sound travels to the wall and back)
- Use speed = distance ÷ time (wave speed equation) to calculate the speed of sound
- Sound waves are longitudinal waves — oscillations/vibrations are parallel to the direction of travel/energy transfer
- Reflection does not change the type of wave — the echo is produced by the sound wave bouncing off the wall, so it remains a longitudinal wave
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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