Page 42 - Q & A STPM 2022 Physics

Physics Semester 3 STPM Chapter 21 Sound Waves
Question 3

A string of length L is stretched between two fixed points. Two successive
resonant frequencies emitted by the string are 315 Hz and 420 Hz. What is the
fundamental resonant frequency of the string?
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A 105 Hz C 210 Hz
B 140 Hz D 280 Hz

Answer: A
Resonant frequencies of a stretched string are: f 0 , 2f 0 , 3f 0 , 4f 0 , 5f 0 , 6f 0 , …
Differences between successive resonant frequencies = 2f 0 - f 0 = f 0 , 3f 0 - 2f 0 = f 0
Therefore fundamental frequency, f 0 = (420 – 315) Hz = 105 Hz

Question 4
Suppose all six equal-length strings of an acoustic guitar are played without
fingering, that is, without being pressed down at any frets. What quantities are
the same for all six strings?
A Fundamental frequency and speed of wave along the strings
B The speed of wave along the strings and the fundamental wavelength
C Fundamental wavelength and speed of sound emitted
D The speed of sound emitted and the fundamental frequency

Answer C
Fundamental wavelength = length of string, which is the same for all the
strings.
Sound travels with the same speed in air irrespective of frequency.
Speed of wave in string, v = T , fundamental frequency, f 0 = 1 T ,
μ 2L μ
mass per unit length of string, μ is different.

Question 5
Which of the following statements about the standing waves in a stretched
string of fixed length is true?
A The mid-point of the string is always a node.
B The mid-point of the string is always an antinode.
C There is always even number of antinodes.
D The total number of nodes and antinodes is always odd.
Semester
3

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021 STPM Q&A Physics T3.indd 402 17/02/2022 10:24 AM