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A sound source A emits waves of frequency 1800 Hz and is moving downward toward the ground with terminal speed v. An observer B standing on the ground directly below the source detects the sound at 2150 Hz. The frequency of the waves reflected back from the ground and received by source A is approximately: (Speed of sound = 343 m/s)

1. 2150 Hz
2. 2500 Hz
3. 1800 Hz
4. 2400 Hz

Correct answer: 2500 Hz

Solution

From f_obs = f*c/(c-v): 2150 = 1800*343/(343-v) gives v ~ 55.8 m/s. The ground receives 2150 Hz and re-emits it; source A (moving toward it) hears 2150*(c+v)/c = 2150*398.8/343 ~ 2500 Hz.

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