A source of sound is allowed to fall freely from the top of a tower of height 20 m. A frequency detector is placed on the ground at the base of the tower. If the frequency of sound emitted by the source is 620 Hz, the apparent frequency recorded by the detector just before the source hits the ground is [velocity of sound in air = 330 m/s and g = 10 m/s2]

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694 Hz

670 Hz

680 Hz

660 Hz

answer is D.

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Detailed Solution

Velocity of the source just before it bits the ground u=2gH=2×10×20 m/s=20 m/s∴ f=f0(1−uv)=620(1−20330)Hz=660 Hz

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