Question

# A source is moving with constant speed vs=20 m/s towards a stationary observer due east of the source. Wind is blowing at the speed of 20 m/s at 60∘ north of east. The source has frequency 1000 Hz. If the speed of sound is 310 m/s, the frequency observed by the observer is approximately

A
1000 Hz
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B
1500 Hz
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C
1067 Hz
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D
1200 Hz
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Solution

## The correct option is C 1067 HzGiven that, Frequency of source, fs=1000 Hz Velocity of source, (vs)=20 m/s From question Velocity of wind along source motion (v′w)=vwcos60∘ =20×12=10 m/s Velocity of sound (v)=310 m/s For apparent frequency, fapp=fs(v+v′wv+v′w−vs) =1000(310+10310+10−20)=1000×320300=1067 Hz Why this question? Caution:- Be careful when taking the sign convention of wind velocity, source velocity and observer velocity in doppler effect.

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