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Question

The pitch of a sound source appears to be 20% decreased to an observer. The velocity of source with respect to the observer will be

A
0.825 ms1
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B
8.25 ms1
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C
82.5 ms1
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D
330 ms1
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Solution

The correct option is C 82.5 ms1
Velocity of source w.r.t observer vsource=vm/s

Velocity of sound in air vsound=330m/s

Let f be the frequency of the source and f be the apparent
frequency heard by the stationary observer when the source moves.

As the apparent frequency decreases, thus the source appears to move away from the observer.

Doppler effect when source moves away from stationary observer:

f=f[vsoundvsound+vsource] =f[330330+v]

Now it is given that f=80 % of ff=0.8f

Thus 0.8f=f[330330+v]

v=82.5m/s

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