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  4. A train is moving on a straight track with speed 20 ms-1. It is blowing its whistle at the frequency of 1000 Hz. The percentage change in the frequency heard by a person standing near the track as the train passes him is close to (speed of sound = 320 ms-1)

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Q. A train is moving on a straight track with speed $20\, ms^{-1}.$ It is blowing its whistle at the frequency of $1000\, Hz$. The percentage change in the frequency heard by a person standing near the track as the train passes him is close to (speed of sound $= 320\,ms^{-1}$)

JEE MainJEE Main 2015Waves

Solution:

$f _{1}=1000\left(\frac{320}{300-20}\right)=1066\, Hz$
$f _{2}=1000\left(\frac{320}{300+20}\right)=941 \,Hz$
$\therefore $ Change is $\simeq 12 \%$