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Physics M.C.Q (1 Marks)

Sound Waves · Rajasthan Board (RBSE) STD 12 Science (Rajasthan - English Medium). 21 questions.

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M.C.Q (1 Marks)

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21 questions · 4 auto-graded MCQ + 17 self-marked written.

Question 11 Mark
A tuning fork of frequency 512Hz is vibrated with a sonometer wire and 6 beats per second are heard. The beat frequency reduces if the tension in the string is slightly increased. The original frequency of vibration of the string is:
  1. 506Hz
  2. 512Hz
  3. 518Hz
  4. 524Hz.
Answer
  1. 506Hz
Explanation:

The frequency of the sonometer may be $512\pm6\text{Hz},$ i.e., $506\text{Hz}$ or $518\text{Hz}.$
On increasing the tension in a sonometer wire, the velocity of the wave (v) increases proportionately as the number of beats decreases. Therefore, the frequency of the sonometer wire is 506Hz.
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Question 21 Mark
Consider the following statements about sound passing through a gas.
  1. The pressure of the gas at a point oscillates in time.
  2. The position of a small layer of the gas oscillates in time.
  1. Both A and B are correct.
  2. A is correct but B is wrong.
  3. B is correct but A is wrong.
  4. Both A and B are wrong.
Answer
  1. Both A and B are correct.
Explanation:
Sound is a longitudinal wave produced by the oscillation of pressure at a point, thus, forming compressions and rarefactions. That portion of gas itself does not move but the pressure variation causes a disturbance.
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MCQ 31 Mark
A small source of sound moves on a circle as shown$-$in figure and an observer is sitting at $0$. Let $v_1, v_2, v_3$ be the frequencies heard when the source is at $A, B$ and $C$ respectively.
  • ✓
    $v_1 > v_2 > v_3$
  • B
    $v_1 = v_2 > v_3$
  • C
    $v_2 > v_3 > v_1$
  • D
    $v_1 > v_2 > v_2.$
Answer
Correct option: A.
$v_1 > v_2 > v_3$

At $B$, the velocity of the source is along the line joining the source and the observer.
Therefore, at $B$, the source is approaching with the highest velocity as compared to $A$ and $C.$
Hence, the frequency heard is maximum when the source is at $B.$
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Question 41 Mark
The speed of sound in a medium depends on:
  1. The elastic property but not on the inertia property.
  2. The inertia property but not on the elastic property.
  3. The elastic property as well as the inertia property.
  4. Neither the elastic property nor the inertia property.
Answer
  1. The elastic property as well as the inertia property.
Explanation:
Propagation of any wave through a medium depends on whether it is elastic and possesses inertia. A wave needs to oscillate (elastic property) for it to be propagated and if it does not have inertia, the oscillations wont keep on moving to and fro about the mean position.
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MCQ 51 Mark
Two sound waves move in the same direction in the same medium. The pressure amplitudes of the waves are equal but the wavelength of the first wave is double the second. Let the average power transmitted across a cross-section by the first wave be $P_1$ and that by the $1$ second wave be $P_2.$ Then,
  • ✓
    $P_1 = P_2$
  • B
    $P_1 = 4P_2$
  • C
    $P_2 = 2P_1$
  • D
    $P_2 = 4P_1$
Answer
Correct option: A.
$P_1 = P_2$
Since the average power transmitted by a wave is independent of the wavelength, we have $P_1 = P_2.$
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Question 61 Mark
An organ pipe, open at both ends, contains
  1. Longitudinal stationary waves.
  2. Longitudinal travelling waves.
  3. Transverse stationary waves.
  4. Transverse travelling waves.
Answer
  1. Longitudinal stationary waves.
Explanation:
An open organ pipe has sound waves that are longitudinal. These waves undergo repeated till resonance to form standing waves.
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Question 71 Mark
The change in frequency due to Doppler effect does not depend on:
  1. The speed of the source.
  2. The speed of the observer.
  3. The frequency of the source.
  4. Separation between the source and the observer.
Answer
  1. Separation between the source and the observer.
Explanation:
$\text{v}_0=\Big(\frac{\text{v}\pm\mu_0}{\text{v}\pm\mu_\text{s}}\Big)\text{v}_\text{B}$
It is clear from the equation that the chage in frequency due to Doppler effect depends only on the relative and not on the distance between the source and the observer.
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Question 81 Mark
An open organ pipe of length L vibrates in its fundamental mode. The pressure variation is maximum.
  1. At the two ends.
  2. At the middle of the pipe.
  3. At distances $\frac{\text{L}}{4}$ inside the ends.
  4. At distances $\frac{\text{L}}{8}$ inside the ends.
Answer
  1. At the middle of the pipe.
Explanation:

For an open pipe in fundamental mode, anti-node is formed at the middle, where the amplitude of the wave is maximum. Hence, the pressure variation is also maximum at the middle.
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Question 91 Mark
A listener is at rest with respect to the source of sound. A wind starts blowing along the line joining the source and the observer. Which of the following quantities do not change?
  1. Frequency.
  2. Velocity of sound.
  3. Wavelength.
  4. Time period.
Answer
  1. Frequency.
  1. Time period.
Explanation:
The frequency does not change. Hence, the time period (inverse of frequency) also remains the same. Due to wind, the relative velocity of sound changes. Thus, the wavelength also changes so as to keep the frequency the same. $(\text{As}\ \nu=\nu\lambda)$
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Question 101 Mark
When we clap our hands, the sound produced is best described by-
  1. $\text{p}=\text{p}_0\sin(\text{kx}-\omega\text{t})$
  2. $\text{p}=\text{p}_0\sin\text{kx}\cos\omega\text{t}$
  3. $\text{p}=\text{p}_0\cos\text{kx}\sin\omega\text{t}$
  4. $\text{p}=\sum\text{p}_{0\text{n}}\sin(\text{k}_\text{n}\text{x}-\omega_\text{n}\text{t}).$
Answer
  1. $\text{p}=\sum\text{p}_{0\text{n}}\sin(\text{k}_\text{n}\text{x}-\omega_\text{n}\text{t}).$
Explanation:
When we clap, there is a change in pressure, which sets a disturbance and forms a wave. However, this variation is not uniform every time we clap (unlike in the case of a sound wave). Hence, we sum up all the disturbances.
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MCQ 111 Mark
The phenomenon of beats can take place:
  • A
    For longitudinal waves only.
  • B
    For transverse waves only.
  • ✓
    For both longitudinal and transverse waves.
  • D
    For sound waves only.
Answer
Correct option: C.
For both longitudinal and transverse waves.
When two or more waves of slightly different frequencies $(v_1 - v_2 >10)$ travel with the same speed in the same direction, they superimpose to give beats.
Thus, the waves may be longitudinal or transverse.
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Question 121 Mark
A source of sound moves towards an observer.
  1. The frequency of the source is increased.
  2. The velocity of sound in the medium is increased.
  3. The wavelength of sound in the medium towards the observer is decreased.
  4. The amplitude of vibration of the particles is increased.
Answer
  1. The wavelength of sound in the medium towards the observer is decreased.
Explanation:
Due to Doppler effect, the frequency or wavelength of the sound changes towards the observer only.
The actual frequency and wavelength of the source does not change.
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Question 131 Mark
When sound wave is refracted from air to water, which of the following will remain unchanged?
  1. Wave number.
  2. Wavelength.
  3. Wave velocity.
  4. Frequency.
Answer
  1. Frequency.
Explanation:
When a sound or light wave undergoes refraction, its frequency remains constant because there is no change in its phase.
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Question 141 Mark
The engine of a train sounds a whistle at frequency v. The frequency heard by a passenger is:
  1. $>\text{v}$
  2. $<\text{v}$
  3. $=\frac{1}{\text{v}}$
  4. $=\text{v}$
Answer
  1. $\text{v}$
Explanation:
For the Doppler effect to occur, there must be relative motion between the source and the observer.
However, this is not the case here. Hence, the frequency heard by the passenger is v.
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Question 151 Mark
When two waves with same frequency and constant phase difference interfere,
  1. There is a gain of energy.
  2. There is a loss of energy
  3. The energy is redistributed and the distribution changes with time.
  4. The energy is redistributed and the distribution remains constant in time.
Answer
  1. The energy is redistributed and the distribution remains constant in time.
Explanation:
The energy is redistributed due to the presence of interference. However, as the frequency and phase remain constant. the distribution also remains constant With time.
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Question 161 Mark
When you speak to your friend, which of the following parameters have a unique value in the sound produced?
  1. Frequency.
  2. Wavelength.
  3. Amplitude.
  4. Wave velocity.
Answer
  1. Wave velocity.
Explanation:
The frequency, wavelength and amplitude do not have a unique value in the sound produced. The frequency (and wavelength) changes as the pitch of the sound varies, while the amplitude is different as the loudness varies. However, the speed of sound in the air at a particular temperature is constant, i.e., it has a unique value.
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Question 171 Mark
A cylindrical tube, open at both ends, has a fundamental frequency v. The tube is dipped vertically in water so that half of its length is inside the water. The new fundamental frequency is:
  1. $\frac{\text{v}}{4}$
  2. $\frac{\text{v}}{2}$
  3. $\text{v}$
  4. $2\text{v}$
Answer
  1. $\text{v}$
Explanation:
If v is the velocity of the wave and L is the length of the pipe. then the fundamental frequency for an open pipe is:
$\text{v}=\frac{\text{v}}{2\text{L}}$
For a closed organ pipe of length of lenth $\text{L}'=\frac{\text{L}}{2},$ the fundamental frequency is:
$\text{v}=\frac{\text{v}}{4\text{L}'}=\frac{\text{v}\times2}{4\times\text{L}}=\frac{\text{v}}{2\text{L}}=\text{v}$
(When the pipe is dipped in water, it behaves like a closed pipe that is half the lenth)
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Question 181 Mark
An electrically maintained tuning fork vibrates with constant frequency and constant amplitude. If the temperature of the surrounding air increases but pressure remains constant, the sound produced will have:
  1. Larger wavelength.
  2. Larger frequency.
  3. Larger velocity.
  4. Larger time period.
Answer
  1. Larger wavelength.
  1. Larger velocity.
Explanation:
The velocity varies with temperature as $\text{v}\propto\sqrt{\text{T}}.$ Therefore, it increases. Since the frequency remains constant, the wavelength will increase as $\lambda\propto\text{v}.$
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MCQ 191 Mark
The fundamental frequency of a vibrating organ pipe is $200\ Hz$.
  • A
    The first overtone is $400\ Hz.$
  • B
    The first overtone may be $400\ Hz$.
  • ✓
    The first overtone may be $600\ Hz.$
  • D
    $600\ Hz$ is an overtone.
Answer
Correct option: C.
The first overtone may be $600\ Hz.$
For an open organ pipe : $\nu_\text{n}=\text{n}\text{v}_1$
$n^{th}$ harmonic $= (n - 1 )^{th}$ overtone
$\nu_1=200\ \text{Hz},\ \nu_2=400\ \text{Hz},\ \nu_3=600\ \text{Hz}$
If the pipe is an open organ pipe, then the $1^{st}$ overtone is $400\ Hz$.
Option $(b)$ is correct.
Also$, \nu_3=600\ \text{Hz},$ i.e., second overtone $= 600\ Hz.$
$600\ Hz$ is an overtone.
Therefore, option $(d)$ is correct.
If the pipe is a closed organ pipe, then $\nu_\text{n}=(2\text{n}-1)\nu_1.$
$(2n - 1)^{th}$ harmonic $= (n - 1)^{th}$ overtone
For $n = 2$
$1^{st}$ overtone$= 3^{rd}$ harmonic $=3\nu_1$
$=3\times200$
$=600\ \text{Hz}$
Therefore, option $(c)$ is also correct.
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Question 201 Mark
The bulk modulus and tbe density of water are greater than those of air. With this much of information, we can say that velocity of sound in air:
  1. Is larger than its value in water.
  2. Is smaller than its value in water.
  3. Is equal to its value in water.
  4. Cannot be compared with its value in water.
Answer
  1. Cannot be compared with its value in water.
Explanation:
If B is the bulk modulus and pis the density, then the velocity of sound is given by:
Velocity $=\sqrt{\frac{\text{B}}{\rho}}$
If both Band p are greater, then we cannot compare $\frac{2\text{B}}{2\rho}=\frac{3\text{B}}{3\rho}=\frac{\text{B}}{\rho}.$
For proper comparison, we need numerical values.
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Question 211 Mark
A tuning fork sends sound waves in air. If the temperature of the air increases, which of the following parameters will change?
  1. Displacement amplitude.
  2. Frequency.
  3. Wavelength.
  4. Time period.
Answer
  1. Wavelength.
Explanation:
The velocity of a sound wave varies with temperature as follows:
$\text{v}\propto\sqrt{\text{T}}$
As the temperature increases, the speed also increases. However, since the frequency remains the same, its wavelength changes.
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