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Question

A train approaching a railway crossing at a speed of 180kmh1 sounds a short whistle at a frequency 600Hz, when it is 400m away from the crossing. The speed of sound in air is 340ms1. 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 680Hz
When 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(vvvscosθ)
Here , vs=180km/h=180×5/18=50m/s
n=600Hz
v=340m/s
From figure , we get
cosθ=4/5
Therefore ,
n=600(34034050×(4/5))
or n=680Hz

620372_586339_ans_4a723c81b9de46fbbaee7036e1427489.png

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