1. Tardigrade
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  4. A train standing at the outer signal of a railway station blows a whistle of frequency 400 Hz in still air. What is the frequency of the whistle for a platform observer when the train recedes from the platform with a speed of 10 ms -1 ? (Speed of sound in air =340 ms -1 )

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Q. A train standing at the outer signal of a railway station blows a whistle of frequency 400 Hz in still air. What is the frequency of the whistle for a platform observer when the train recedes from the platform with a speed of $10 \,ms ^{-1}$ ? (Speed of sound in air $=340 \,ms ^{-1} )$

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Solution:

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The train is the source and the platform observer is the listener.
$v_{s}=-10 ms ^{-1}, v_{L}=0 ; v=340 m s ^{-1}$
$v =\left(\frac{v-v_{L}}{v-v_{S}}\right) v_{0}$
$=\left(\frac{v-v_{L}}{v-v_{S}}\right) 400 Hz =389 Hz$