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The MCQs below are drawn from the Physics subject category.
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871
In space, sound waves cannot travel because there is:
A. Light
B. Air
C. Gravity
D. Vacuum
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Correct Option
Explanation
Sound requires a material medium to propagate, and space is essentially a vacuum, so sound cannot travel there.
872
A person speaking normally produces a sound intensity of 40 dB at a distance of 1 m. If the threshold intensity for reasonable audibility is 20 dB, the maximum distance at which he can be heard clearly is
A. 5 m
B. 20 m
C. 10 m
D. 4 m
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Explanation
A 20 dB drop corresponds to intensity ratio 100, so distance increases by √100 = 10; from 1 m to 10 m (option C).
873
To produce beats it is necessary to use two waves
A. of equal amplitudes
B. of equal wavelengths
C. travelling in opposite direction
D. of slightly different frequencies
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Explanation
Beats arise from interference of two waves with slightly different frequencies, producing periodic variations in amplitude.
874
Sounds of frequencies higher than 20,000 Hz are called
A. supersonics
B. infrasonic
C. ultrasounds
D. audible sound waves
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Explanation
Sounds above about 20 kHz are ultrasonic (ultrasounds), above the human hearing range.
875
An astronaut can't hear his companion at the surface of the moon because
A. there is no medium for sound propagation
B. produced frequencies are above the audio-frequencies
C. temperature is too low during night and too high during day
D. there are too many craters on the surface of the moon
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Explanation
Sound requires a material medium to propagate and the Moon has essentially no atmosphere to carry sound.
876
Two aeroplanes are moving towards each other. One of them blows a horn. The person in the other plane hears the sound. The apparent pitch
A. becomes zero
B. decreases
C. increases
D. becomes infinite
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Correct Option
Explanation
When source and observer move towards each other, the Doppler effect increases frequency, so the apparent pitch increases.
877
With what velocity must a listener move away from the stationary source so that the apparent frequency noted by him is one third of the real frequency? The velocity of sound in air is 333 m/s.
A. 332 m/s
B. 222 m/s
C. 166 m/s
D. 183 m/s
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Explanation
For observer moving away, f' = f( (v - v_o)/v ) = f/3 → v_o = v(1 - 1/3) = 2v/3 = 2×333/3 = 222 m/s.
878
Speed of sound in vacuum (in m/sec) is:
A. 156
B. 330
C. Zero
D. 1000
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Explanation
There is no medium in vacuum to carry sound, so the speed of sound in vacuum is effectively zero.
879
The wavelength of sound from a tuning fork of frequency 330 hertz is nearly
A. 330 cm
B. 10 cm
C. 1 cm
D. 100 cm
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Explanation
Using v ≈ 330 m/s, wavelength λ = v/f = 330/330 = 1 m = 100 cm.
880
Two organ pipes produce 7 beats per second at 5°C. When the temperature rises to 10°C, the number of beats is
A. Data is inadequate
B. > 7
C. = 7
D. < 7
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Correct Option
Explanation
Frequencies of both pipes increase with speed of sound (which rises with temperature); their difference (beat frequency) scales up, so the number of beats increases (>7).
