A student measures the speed of sound in air by standing a large distance from a tall brick wall and clapping their hands at a steady rhythm until each clap coincides with the echo of the previous clap. Explain how the student can use this method to calculate the speed of sound in air, and state what two quantities must be measured.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The student stands 85 m from the wall and adjusts their clapping rate until each clap perfectly coincides with the returning echo of the previous clap.
Model answer (4 marks)
The sound produced by a clap travels to the wall and is reflected back as an echo. The total distance travelled by the sound in one clap–echo cycle is therefore twice the distance from the student to the wall. The student measures this distance, d, and the time, t, taken for one clap–echo cycle (the interval between successive claps). The speed of sound, v, is then calculated from the definition of speed:
v = (total distance)/(time) = (2d)/t.
v = (total distance)/(time) = (2d)/t.
Examiner tips
- Use the definition of speed (distance ÷ time) in your calculation. Mention that the total distance is 2× the student–wall distance. State clearly the two measured quantities: distance to wall and time of one cycle. Show the final formula v = 2d/t to demonstrate understanding.
Common mistakes
- Confusing the time between claps with the time for the echo to return; students often use the wrong time interval. Forgetting to double the distance to the wall. Using the wrong units or not showing the calculation step.
Mark scheme (4 marks)
- The sound (clap) travels to the wall and is reflected back as an echo, so the total distance travelled by the sound is twice the distance from the student to the wall.
- The student must measure the distance from the wall.
- The student must measure the time for one clap–echo cycle (the time period between successive claps).
- Speed is calculated by dividing the total distance (2 × distance to wall) by the time for one cycle, using speed = distance ÷ time.
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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