First slide

Doppler

Question

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 $m / s^{2}$ ]

Moderate

A

694 Hz
B

670 Hz
C

680 Hz
D

660 Hz

Solution

Velocity of the source just before it bits the ground
$u = \sqrt{2 g H} = \sqrt{2 \times 10 \times 20} m / s = 20 m / s$
$∴ f = \frac{f_{0}}{(1 - \frac{u}{v})} = \frac{620}{(1 - \frac{20}{330})} H z = 660 H z$

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