A train, standing in a station-yard, blows a whistle of frequency $400\,Hz$ in still air. The wind starts blowing in the direction from the yard to the station with a speed of $10\,ms^{-1}$ then value of frequency and speed of sound for observer will be (the speed of sound in still is $340\, ms^{-1}$ )
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Here, $v_{0}=400 H z, v=340 m s^{-1}, v_{w}=10 m s^{-1}$
As there is no relative motion between the source and observer, therefore the frequency of sound remains unchanged.
Hence, the frequency of sound as heard by an observer standing on the platform is $v_{0}=400 H z$
As the wind is blowing in the direction of sound, therefore, for an observer standing on platform,
Speed of sound $=v+v_{w}=340+10=350 \mathrm{ms}^{-1}$
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