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Q.

A train approaching a hill a speed of 40km/hr sounds a whistle of frequency 580 Hz when it is at a distance of 1 km from a hill. A wind with a speed of 40 km/hr is blowing in the direction of motion of the train. Find the frequency (in Hz) of the whistle as heard by an observer on the hill. (velocity of sound in air = 1200 km/hr)

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answer is 599.

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Detailed Solution

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According to Doppler’s effect, the apparent frequency when both source and observer move along the same direction is
n'=(v+w)v0(v+w)vs
Velocity of observer    v0=0                n'=(v+w)v+wvsn        
Given  v=1200km/hr,w=40km/hr,vs=40km/hr,and    n=580Hz 
n'=1200+40(1200+40)40×580=599.33Hz=600Hz.

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