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CAIE A-Level Physics AS 7.3 Doppler Effect for Sound Waves Questions

Practise explaining and calculating Doppler frequency shifts for moving sound sources, selecting the correct geometry and sign.

Syllabus
2028–2030
Course
Physics 9702
Level
AS

Exam points

  • explain qualitatively how relative source–observer motion changes observed sound frequency and determine the direction of the change
  • calculate and interpret observed frequency with fₒ = fₛv/(v ± vₛ), selecting the correct geometry and sign

Covered syllabus points

7.3.1—When a source of sound waves moves relative to a stationary observer, the7.3.2—The expression fο = f sv / (v ± vs) for the observed frequency when a source

Question 1

[Maximum number: 1]

An ambulance siren emits a sound with a single frequency f.
The ambulance travels towards, passes close to, and then travels away from a stationary observer.

Which statement describes the frequency of the sound detected by the observer as the ambulance passes the observer?

A

equal to f and decreasing

B

equal to f and increasing

C

greater than f and constant

D

less than f and constant

Question 2

[Maximum number: 1]

The siren of a moving police car emits a sound wave with a frequency of 440 Hz . A stationary observer hears sound of frequency 494 Hz . The speed of sound in the air is 340 ms−1340 \mathrm{~ms}^{-1}.

What could be the speed and the direction of movement of the car?

A

37 m s−137 \mathrm{~m} \mathrm{~s}^{-1} directly away from the observer

B

37 m s−137 \mathrm{~m} \mathrm{~s}^{-1} directly towards the observer

C

42 m s−142 \mathrm{~m} \mathrm{~s}^{-1} directly away from the observer

D

42 m s−142 \mathrm{~m} \mathrm{~s}^{-1} directly towards the observer

Question 3

[Maximum number: 1]

A man stands stationary in front of a swing. A child sits and swings.

Figure for Question 3 — CAIE A-Level Physics AS

The child blows a whistle that emits a sound at a constant frequency.
The man observes the frequency of the sound when the swing is at positions X, Y and Z .
When will the man hear the highest frequency?

A

when the swing is at X

B

when the swing is at Y and moving away from the man

C

when the swing is at Y and moving towards the man

D

when the swing is at Z

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