Page 59 - Q & A STPM 2022 Physics

Physics Semester 3 STPM Chapter 21 Sound Waves
Question 4

A source of sound has a constant frequency. If all the motions described below
have the same speed, the observed frequency of sound is the highest in
A the source and observer are moving away from each other
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B the source and observer are moving toward each other
C the source is moving toward a stationary observer
D the source is stationary and the observer is moving toward it

Answer: B
The separation between the source and observer decreases at the fastest rate
when they are moving toward each other.

Question 5

A submarine that emits sonar of frequency f is moving with a velocity u toward
a stationary object. The speed of sonar in water is v. The frequency of the sonar
reflected from the object and detected by the submarine is
A  v – u  f
v + u
B  v  f
v – u
C  v + u  f
v – u
D  v + u  f
v

Answer: C
Use the equation f’ =  v  u o  f
v  u s
v
Wave incident on stationary object, f 1 =  v – u  f
For the reflected wave, the submarine is the “observer”
v
f

frequency detected, f 2 =  v + u  1 =  v + u   v – u  f
v
v
=  v + u  f Semester
v – u
3

419

021 STPM Q&A Physics T3.indd 419 17/02/2022 10:24 AM