Page 61 - Q & A STPM 2022 Physics

Physics Semester 3 STPM Chapter 21 Sound Waves

Section C Essay Question
Question 7

(a) What is meant by Doppler Effect?
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(b) A sound source emits sound of constant frequency. Draw three circles
to represent three successive wavefronts from the source in each of the
following situations.
(i) The source is stationary.
(ii) The source moves toward an observer with a speed less than the speed
of sound.
(iii) The source moves toward an observer with a speed that equals to the
speed of sound.
(c) From the diagrams in (b)(ii), deduce the changes in the wavelength and
frequency detected by the observer.
(d) Explain why a loud sound is heard by the observer in (b)(iii).
(e) An ambulance moving with a velocity of 42 m s sounds its siren of
–1
frequency 500 Hz. A car is moving in the same direction as the ambulance
–1
–1
at 25 m s . The speed of the sound is 330 m s .
What frequency does a person in the car hear
(i) as the ambulance approaches the car?
(ii) after the ambulance passes the car?

Answer
(a) Doppler effect: The change in the frequency of sound heard by an
observer when there is relative motion between the sound source and the
observer.

(b)

0
<
u
S S u < v v S
u=v
(i) (ii) (iii)

(c) The wavelength is shorter. The frequency increases.
(d) The wavefronts overlap. Constructive superposition that results in the
sound wave received by the observer has a high amplitude. Semester

3

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021 STPM Q&A Physics T3.indd 421 17/02/2022 10:24 AM