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Question:

A source of sound A emit waves of frequency 1800 Hz is falling towards ground with a terminal speed v. The observer B on the ground directly beneath the source receives waves of frequency 2150 Hz. The source A receives waves, reflected from ground, of frequency nearly :(Speed of sound=343 m/s)

2150Hz

2500Hz

2400Hz

1800Hz

Solution:

From Doppler's effect, we know
fo=(v+vo)/(v+vs)fs
343/(343+vs)=2150/1800 as vo=0 (i)
Now reflected wave will become source with velocity vs and frequency 2150Hz and A will become observe. Therefore,
f'o=343/(343+vs) × 2150 (ii)
From i and ii
f'o=2150 × 2150/1800=2568Hz
Frequency is approx 2500Hz.