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Physics [5 Mark Question Answer]

Sound · ICSE STD 10 (ICSE - English Medium). 3 questions.

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[5 Mark Question Answer]

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Question 15 Marks
(i) (a) Name the characteristic of sound which enables a person to differentiate between two sounds with equal loudness but having different frequencies.
(b) Define the chanacteristic named by you in (a).
(c) Name the characteristic of sound which enables a person to differentiate between two sounds of the same loudness and frequency but produced by different instruments.
(ii) (a) (i) What are mechanical waves?
(ii) Name one property of waves that do not change when the wave passes from one medium to another.
(b) The diagram shows three different modes of vibration P, Q and R of the same string:
(i) Which vibration will produce a louder sound and why?
(ii) The sound of which vibration will have maximum shrillness?
(iii) State the ratio of wavelengths P and R .
Image
(iii) Write down differences between free vibration and forced vibration.
Answer
(i) (a) The characteristic is called pitch of sound.
(b) The sensation produced in the brain due to the sound of same loudness, but different frequencies are called pitch of sound. Generally, lower frequencies produce bass effect and higher frequencies shrill effect.
(c) The characteristic is called timbre or quality of sound. It depends upon the waveform of the sound.
(ii) (a) (i) The waves produces in matter (solid, liquid or gas) are called mechanical waves.
(ii) The frequency of the waves does not change on passing from one medium to another.
(b) (i) R will produce a louder sound, because loudness increases with the increase in amplitude.
(ii) P will have maximum shrillness, because shrillness increases with the increase in frequency.
(iii) Ratio of wavelengths of P and R is $1: 3$,
(iii) Differences between the Free and Forced vibrations
Image
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Question 25 Marks
(i) How free vibration is different from damped vibration?
(ii) An enemy plane is at a distance of 300 km from a radar. In how much time the radar will be able to detect the plane? Take velocity of radio waves as $3 \times 10^5 kms ^{-1}$.
(iii) (a) (i) Draw a graph between displacement and the time for a body executing free vibrations.
(ii) Where can a body execute free vibrations?
(b) (i) State the safe limit of sound level in terms of decibel for human hearing.
(ii) Name the characteristic of sound in relation to its waveform.
Answer
(i) Differences between the Free and Damped vibrations
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(ii) Given, $d=300 km=3 \times 10^5 m$; Velocity of the radio waves $=3 \times 10^5 kms ^{-1}=3 \times 10^8 ms^{-1}$
$\therefore t=\frac{2 d}{v}=\frac{2 \times 3 \times 10^5}{3 \times 10^8}=2 \times 10^{-3} s$
(iii) (a) (i)
Image
(ii) Free vibrations can be executed in vacuum only.
(b) (i) Human ears can safely pick up sound from 50 dB to 60 dB .
(ii) Quality of sound or Timbre. 
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Question 35 Marks
(i) When a tuning fork, struck by a rubber pad, is held over a length of air column in a tube, it produces a loud sound for a fixed length of the air column.
(a) Name the above phenomenon.
(b) How does the frequency of the loud sound compare with that of the tuning fork?
(c) Name the unit for measuring the loudness of sound.
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(ii) (a) Displacement-distance graph of two sound waves $A$ and B , travelling in a medium, are as shown in the diagram below.
Study the two sound waves and compare their :
(i) Amplitudes,
(ii) Wavelengths.
(b) (i) What do you understand by free vibrations of a body?
(ii) Why does the amplitude of a vibrating body continuously decrease during damped vibrations?
(iii) A man stands in front of a cliff and fires a gun. He hears an echo after 4 seconds. On moving closer to the cliff by 41.25 m and again firing a gun, he hears an echo after 3.75 s . Calculate :
(a) speed of sound
(b) distance of the man from the cliff to his initial position.
Answer
(i) (a) The phenomenon is called resonance.
(b) Tuning fork as well as loud sound have same frequency.
(c) Decibel ( dB ) is the unit for measuring loudness.
(ii) (a) (i) Ratio of amplitude of A: Amplitude of $B =2: 1$
(ii) Ratio of the wavelenght of $A$ : Wavelength $B=1: 2$
(b) (i) The periodic vibrations of a body of constant amplitude in the absence of any external force are called free vibrations of a body.
(ii) The amplitude of a vibrating body continuously decreases during damped vibrations due to the presence of resistive force. So, the vibrating body loses energy, with the result that its amplitude gradually decreases, till it comes to rest.
(iii) Let $x m$ be the initial distance of the man from the cliff and $V$ the speed of sound.
Case (i) $t=\frac{2 d}{V}$
$\therefore 4-\frac{2 x}{V}$ or $V=\frac{x}{2}$...(i)
Image
$\begin{aligned} \text { Case (ii) } t & =\frac{2 d}{V} \\ 3.75 & =\frac{2(x-41.25)}{V}\end{aligned}$
or $ V=\frac{2 x-82.5}{3.75}$...(ii)
Comparing (i) and (ii)
$\frac{x}{2}=\frac{2 x-82.5}{3.75} \Rightarrow 3.75 x=4 x-165$
$\therefore x=660 m .$
Substituting the value of $x$ in (i)
$V=\frac{660}{2}=330 ms^{-1}$
(a) Speed of sound $= 3 3 0 ms ^{-1}$
(b) Distance of the man from the cliff to his initial position $=660 m$
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[5 Mark Question Answer] - Physics STD 10 Questions - Vidyadip