Page 57 - Q & A STPM 2022 Physics

Physics Semester 3 STPM Chapter 21 Sound Waves

(iii) By what percentage should the piano tuner change the tension in the
string to bring it into tune?
State whether the change is an increase or decrease.

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Answer
(a) Beats are the periodic and repeating fluctuations in the intensity of a
sound when two sound waves of very similar frequencies interfere with
one another.
Conditions: Sound waves of slight difference in frequencies, and equal or
approximately equal amplitudes.

(b) (i) f 2 – 523 = 3.00 Hz, or 523 – f 2 = 3.00 Hz
f 2 = 526 Hz or f 2 = 520 Hz

(ii) f 2 = 1 T when T is increased, f 2 increased.
2L μ
Beat frequency increases to 4.00 Hz.
Therefore correct equation is f ' 2 – 523 = 4.00 Hz
Hence new frequency of string f ' 2 = 527 Hz

1 1 T
(iii) f ' 2 = T' 2 = 527 Hz , f = = 523 Hz
2L μ 2L μ
T 523 2
=  
T' 2 527
T = 0.985 T' 2
Tension in the string should be reduced by 1.5%.

21.5 Doppler Effect

Section A Multiple-choice Questions

Question 1
–1
A police car with its siren on travelled at a speed of 20 m s toward an observer.
The frequency of the siren is 900 Hz. What is the frequency heard by the Semester
–1
observer? (Speed of sound in air is 330 m s ).
A 845 Hz C 954 Hz
B 849 Hz D 958 Hz 3

417

021 STPM Q&A Physics T3.indd 417 17/02/2022 10:24 AM