Page 58 - Q & A STPM 2022 Physics

Physics Semester 3 STPM Chapter 21 Sound Waves
Answer: D
Frequency increases if separation between observer and source decreases and
decreases when the separation increases.
Apparent frequency, f ’ =  v  f
v – u s
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330
=  330 – 20 (900) Hz
= 958 Hz
Common Error
Do not confure the velocity u 0 of the observer, and u s the velocity of the source
in the equation apparent frequency, f ’ =  v ± u 0  f.
v  u s
If the observer and the source are getting closer, f’ > f, and if the separation
between them is increasng, f’ > f.

Question 2

An observer moves with a constant speed away from a stationary source that
emits sound of frequency f and wavelength λ. The sound detected by the
observer has frequency and wavelength
Frequency Wavelength
A less than f equals λ
B greater than f less than λ
C greater than f equals λ
D less than f greater than λ

Answer: A
When the observer moves away from the source, the frequency detected is
lower because the speed of sound relative to the observer decreases.
Question 3
A sound source of frequency 1.00 kHz moves at a speed of 30.0 m s toward
–1
a listener who moves at a speed of 20.0 m s in a direction away from the
–1
source. What is the apparent frequency heard by the listener? (Speed of sound
–1
= 330 m s )
A 857 Hz B 861 Hz C 1033 Hz D 1129 Hz

Answer: C
Frequency heard by the listener, f’ =  v – u o  f
Semester
v – u s
3 =  330 – 20.0  1000 Hz
330 – 30
= 1033 Hz

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021 STPM Q&A Physics T3.indd 418 17/02/2022 10:24 AM