Doppler effect for sound waves: past paper questions
Real questions on doppler effect for sound waves
The newest 20 of 20 written questions. Open one to see it on the real paper.
- 9702/21 · May/June 2025 · Q5b(i)[2]
Calculate the frequency of the sound waves detected by the microphone. frequency = Hz
See it on the paper → - 9702/21 · May/June 2025 · Q5b(ii)[2]
Determine the speed of the sound in air. speed of sound = m s −1
See it on the paper → - 9702/22 · February/March 2025 · Q5b[2]
The loudspeaker now moves in a straight line while emitting the same sound of constant frequency. The period of the trace on the CRO increases continuously. Describe the motion of the loudspeaker.
See it on the paper → - 9702/42 · February/March 2025 · Q10b(ii)[2]
State and explain how the expected wavelength of the light observed from Z compares with the emitted wavelength.
See it on the paper → - 9702/23 · October/November 2024 · Q5b(i)[2]
The speed of sound in air is 320 m s –1 . maximum frequency = Hz Calculate the maximum frequency of the sound detected by the observer.
See it on the paper → - 9702/23 · October/November 2024 · Q5b(ii)[2]
At time t = 0, the observer detects the sound emitted by the source when it was in the position shown in Fig. 5.2. On Fig. 5.3, sketch the variation with t of the frequency f of the sound detected by the observer for one complete rotation of the platform. Calculations are not required. f 0 t
See it on the paper → - 9702/42 · May/June 2024 · Q8a(i)[2]
Explain how the positions of the lines in the emission spectrum seen by an observer on the Earth differ from the positions shown in Fig. 8.1.
See it on the paper → - 9702/42 · May/June 2024 · Q8a(ii)[2]
On Fig. 8.1, draw the three lines in possible positions in the spectrum seen by the observer. (ii)
See it on the paper → - 9702/22 · February/March 2022 · Q5b(i)[2]
Calculate the frequency, to three significant figures, of the sound emitted by the horn. frequency = Hz
See it on the paper → - 9702/22 · February/March 2022 · Q5b(ii)[1]
The motorcycle rider passes the stationary pedestrian and then moves directly away from her. As the rider moves away, he operates the horn for a second time. The pedestrian now hears sound that is increasing in frequency. State the variation, if any, in the speed of the motorcycle when the rider operates the horn for t...
See it on the paper → - 9702/21 · October/November 2021 · Q4b(i)[2]
Explain why the frequency of the sound detected by the observer is sometimes above and sometimes below 1.2 kHz.
See it on the paper → - 9702/21 · October/November 2021 · Q4b(ii)[1]
State the name of the phenomenon in (b)(i).
See it on the paper → - 9702/21 · October/November 2021 · Q4c[3]
The speed of the sound in the air is 320 m s –1 . Use Fig. 4.2 to determine the speed of the vehicle in (b). speed = m s –1
See it on the paper → - 9702/22 · October/November 2021 · Q4b[2]
Determine the frequency, to three significant figures, of the sound heard, by the child, that was emitted from the car horn at time t = 3.0 s. frequency = Hz
See it on the paper → - 9702/23 · October/November 2021 · Q3d[3]
The aircraft in (c) travels from X to Y in a time of 14 s. Fig. 3.3 shows that, as the aircraft travels from X to Y, it moves directly towards an observer who is standing on the ground. aircraft X 280 m Y observer ground Fig. 3.3 (not to scale) The aircraft emits sound as it travels from X to Y. The observer hears soun...
See it on the paper → - 9702/22 · February/March 2020 · Q2a[2]
The dolphin emits a sound as it swims directly towards a stationary submerged diver. The frequency of the sound heard by the diver is 9560 Hz. The speed of sound in the water is 1510 m s –1 . Determine the frequency, to three significant figures, of the sound emitted by the dolphin. frequency = Hz
See it on the paper → - 9702/21 · October/November 2019 · Q3f[2]
When the aircraft is hovering at a high altitude, an electric fault causes the propellers to stop rotating. The aircraft falls vertically downwards. When the aircraft reaches a constant speed of 22 m s –1 , it emits sound of frequency 3.0 kHz from an alarm. The speed of the sound in the air is 340 m s –1 . Determine th...
See it on the paper → - 9702/22 · May/June 2019 · Q2d[2]
The driver of the car in (b) sees a pedestrian standing directly ahead in the distance. The driver operates the horn of the car from time t = 15 s to t = 17 s. The frequency of the sound heard by the pedestrian is 480 Hz. The speed of the sound in the air is 340 m s −1 . Use Fig. 2.2 to calculate the frequency of the s...
See it on the paper → - 9702/41 · May/June 2019 · Q4a(i)[1]
attenuation
See it on the paper → - 9702/41 · May/June 2019 · Q4a(ii)[2]
noise.
See it on the paper →
What examiners said
Many candidates recognised that the loudspeaker was moving away. Many also gave correct explanations relating the increase in period to a decrease in frequency, although this was not required as the command for this question was ‘describe’ not ‘explain’. A small minority realised that the continuous increase in the period meant that the speed of the loudspeaker was increasing. A very small number...
— Examiner report, February/March 2025, 9702/22 Q5(b) (Cambridge)
Many candidates incorrectly asserted that the observed wavelength will be larger due to redshift. The stronger candidates realised that point Z on the Sun is moving towards the observer, and hence the observed wavelength in this situation will be less than the emitted wavelength.
— Examiner report, February/March 2025, 9702/42 Q10(b)(ii) (Cambridge)
Almost all candidates tried to apply the Doppler effect formula given on the Formulae Sheet. A significant number of candidates made mistakes when substituting the data into this algebraic formula. The value of the observed frequency was often inappropriately substituted as the source frequency. Many candidates did not give their answer to three significant figures as instructed in the question.
— Examiner report, February/March 2022, 9702/22 Q5(b)(i) (Cambridge)
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