CAIE A-Level Physics 7.3 Doppler Effect for Sound Waves

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

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, the 7.3.2—The expression fο = f sv / (v ± vs) for the observed frequency when a source

[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)

The sledge passes point A with speed $18 \mathrm{~ms}^{-1}$ 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 $t=24 \mathrm{~s}$ is shown in Fig. 3.2.

Fig. 3.2

(a)

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

Use Fig. 3.2 to

[ 3 ]

(i)

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

[ 2 ]

$f_{0}=(900 \times 340) /(340+12)$

C1

$=870 \mathrm{~Hz}$

A1

(ii)

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

[ 1 ]

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Covered syllabus points

7.3 Doppler effect for sound waves question 1

Question (a)

Question (i)

Question (ii)