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Question

# A train approaching a railway crossing at a speed of 180kmhâˆ’1 sounds a short whistle at a frequency 600Hz, when it is 400m away from the crossing. The speed of sound in air is 340msâˆ’1. The frequency of the sound heard by a person standing on a road perpendicular to the track at a distance of 300m from the crossing is:

A
680Hz
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B
480Hz
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C
40Hz
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D
50Hz
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Solution

## The correct option is A 680HzWhen a source is moving in a direction making an angle w.r.t. a stationary observer , then apparent frequency heard by observer is given by , n′=n(vv−vscosθ)Here , vs=180km/h=180×5/18=50m/s n=600Hz v=340m/sFrom figure , we get cosθ=4/5Therefore , n′=600(340340−50×(4/5))or n′=680Hz

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