CourseCourseConceptConceptDocsDocuments

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: 3]

A child on a sledge slides down a steep hill and then travels in a straight line up an ice-covered slope, as illustrated in Fig. 3.1.

Fig. 3.1 (not to scale)

Fig. 3.1 (not to scale)

The sledge passes point A with speed 18 ms−118 \mathrm{~ms}^{-1} at time t=0 and then comes to rest at point B. The child applies a brake to the sledge at point B. The brake does not keep the sledge stationary and it immediately slides back down the slope towards A .

The variation with time t of the velocity v of the sledge from t=0 to t=24 st=24 \mathrm{~s} is shown in Fig. 3.2.

Fig. 3.2

Fig. 3.2

Question (a)

(a)

The child on the sledge blows a whistle between t=4.0 st=4.0 \mathrm{~s} and t=8.0 st=8.0 \mathrm{~s}. The whistle emits sound of frequency 900 Hz . The speed of the sound in the air is 340 ms−1.A340 \mathrm{~ms}^{-1} . \mathrm{A} man standing at point A hears the sound.

Use Fig. 3.2 to

[ 3 ]

Question (i)

(i)

determine the initial frequency of the sound heard by the man,
initial frequency = Hz

[ 2 ]

Question (ii)

(ii)

describe and explain qualitatively the variation, if any, in the frequency of the sound heard by the man.

[ 1 ]

Question 2

[Maximum number: 3]

The sound wave emitted from the horn of a stationary car is detected with a microphone and displayed on a cathode-ray oscilloscope (c.r.o.), as shown in Fig. 5.1.

Fig. 5.1

Fig. 5.1

The y-axis setting is 5.0mVcm−15.0 \mathrm{mV} \mathrm{cm}^{-1}.
The time-base setting is 0.50 ms cm−10.50 \mathrm{~ms} \mathrm{~cm}^{-1}.

The horn of the car sounds continuously. Describe the changes to the trace seen on the c.r.o. as the car travels at constant speed
1. directly towards the stationary microphone,
2. directly away from the stationary microphone.

All question bank results loaded