Page 55 - Q & A STPM 2022 Physics

Physics Semester 3 STPM Chapter 21 Sound Waves
(c) (i) AC = 5.00 m
P 2.50 × 10 –3
–2
I A = = W m = 7.96 μW m –2
4π(AC) 2 4π(5.00) 2
–12 
b A = 10 log  7.96 × 10 –6 = 69.0 dB
10
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(ii) BC = 2.00 + 4.00 = 18.00 m 2
2
2
2
P 4.50 × 10 –3
–2
I B = = W m = 19.9 μW m –2
4π(BC) 2 4π(18.00)
–12 
b B = 10 log  19.9 × 10 –6 = 73.0 dB
10
(iii) b A+B = 10 log  (7.96 + 19.9) × 10 –6  = 74.4 dB
–12
10
21.4 Beat

Section A Multiple-choice Questions

Question 1
Two tuning forks are sounded at the same time, 30 beats are heard in 5 seconds.
If one of the tuning forks has a frequency of 252 Hz, what are the possible
frequencies of the other tuning fork?
A 246 Hz or 258 Hz C 249 Hz or 255 Hz
B 247 Hz or 257 Hz D 237 Hz or 267 Hz

Answer: A
Beat frequency = 30 Hz = 6 Hz
5
f 2 – 252 = 6 Hz, or 252 – f 2 = 6 Hz
f 2 = 258 Hz or f 2 = 246 Hz

Question 2
A tuning fork of frequency 262 Hz is sounded along with a guitar string. Four
beats are heard every second. Next, a bit of wax is melted onto the tine of the Semester
tuning fork. The tuning fork now produces five beats per second with the same
guitar string. What is the frequency of the string?
A 257 Hz C 262 Hz 3
B 258 Hz D 266 Hz

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021 STPM Q&A Physics T3.indd 415 17/02/2022 10:24 AM