Page 56 - Q & A STPM 2022 Physics

Physics Semester 3 STPM Chapter 21 Sound Waves
Answer: D
Beat frequency, 4 Hz = (262 – f) Hz, or 4 Hz = (f – 262) Hz
With wax on its tine, frequency of tuning fork is less than 262 Hz.
Beat frequency increases to 5 Hz, hence the correct equation is
4 Hz = (f – 262) Hz
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Frequency of string, f = (262 + 4) Hz = 266 Hz

Section B Structured Questions

Question 3

Two identical strings of length 0.700 m have the same fundamental frequency of
440 Hz. The tension in one of the strings is then increased by 2.0%. The strings
are now struck, what is the beat frequency between the fundamentals of the
two strings?

Answer
T
The speed of wave in a stretched string, v = where T = tension, μ = mass m –1
μ
v 1 T
Fundamental frequency, f = =
2L 2L μ
T 1 = 1.020T
Therefore f 1 = 1.020 f
= 1.020(440 Hz)
= 444 Hz

Beat frequency, f beat = f 2 – f 1
= (444 – 440) Hz
= 4.0 Hz

Section C Essay Question

Question 4
(a) Explain the meaning of beats. State the conditions for the beats to be heard.
(b) When the note C of a piano at 523 Hz is tuned using a reference oscillator,
–1
3.00 beats s are heard.
Semester
3 (i) What are the possible frequencies of the string?
(ii) When the string is tightens slightly, 4.00 beats s are heard. What is the
–1
frequency of the string?
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021 STPM Q&A Physics T3.indd 416 17/02/2022 10:24 AM