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The MCQs below are drawn from the Physics subject category.
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851
A person moves with a speed thrice the speed of sound waves towards the stationary source of sound. Then the frequency of sound waves heard by the person will:
A. Increase 4 times
B. Increase 3 times
C. Decrease 4 times
D. Decrease 3 times
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Explanation
Observer moving toward the source at 3v hears frequency f' = f(1 + vo/v) = f(1+3) = 4f, i.e., increases four times.
852
Sound waves do not travel in vacuum because
A. they require material medium for propagation
B. they are transverse waves
C. they are stationary waves
D. they do not have enough energy
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Explanation
Sound is a mechanical wave and needs a material medium (air, liquid, solid) to propagate, so it cannot travel in vacuum.
853
The unit of intensity level is
A. watt
B. sabin
C. bel
D. joule
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Explanation
Intensity level (sound level) is commonly measured in bels (or decibels), so bel is the unit for intensity level.
854
The loudness of a sound depends on its
A. wave amplitude
B. regularity
C. wavelength
D. frequency
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Explanation
Loudness is determined primarily by the amplitude of the sound wave; larger amplitude is perceived as louder.
855
The frequency range for human ear is:
A. 20 KHz-20,000 KHz
B. 20-20,000 Hz
C. 20-20,000 MHz
D. None of these
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Explanation
The typical audible range for humans is about 20 Hz to 20,000 Hz.
856
Of the material mentioned below the speed of sound is largest in
A. vacuum
B. steel
C. air
D. water
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Explanation
Sound travels fastest in solids (like steel) because they have higher elastic modulus and can transmit vibrations more rapidly.
857
Increase in velocity of sound in air for 1℃ rise in temperature is
A. 61.0 m s^-1
B. 0.61 m s^-1
C. 1.61 m s^-1
D. 2.00 m s^-1
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Explanation
The approximate increase in speed of sound in air is about 0.6 m/s per degree Celsius, so 0.61 m/s is correct.
858
A pendulum vibrates with a time period of 1 s. Which range of sound is produced by it?
A. Ultrasonic
B. Infrasonic
C. Supersonic
D. Audible
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Explanation
A period of 1 s corresponds to frequency 1 Hz, which is below the audible range (<20 Hz), so it is infrasonic.
859
A star emitting yellow light starts moving towards Earth. Its colours as seen from Earth will turn gradually:
A. Blue
B. Orange
C. Red
D. Dark
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Explanation
Motion towards the observer causes blueshift, so yellow light will shift towards blue as the star approaches.
860
What is shape of a pure note?
A. Sawtooth
B. Triangular wave
C. Sine wave
D. Square wave
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Explanation
A pure musical note corresponds to a single sinusoidal oscillation, so its waveform is a sine wave.
