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 )

Question

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 ) (A) 366 Hz (B) 389 Hz (C) 400 Hz (D) 412 Hz

Tardigrade Admin · Accepted Answer

<img class="img-fluid question-image" alt="image" src="https://cdn.tardigrade.in/img/question/physics/a141ffc3df4e5536319f8fdc53984a2d-.png" /> The train is the source and the platform observer is the listener. vs=-10 ms -1, vL=0 ; v=340 m s -1 v =((v-vL/v-vS)) v0 =((v-vL/v-vS)) 400 Hz =389 Hz

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 $10 m s^{- 1}$ ? (Speed of sound in air $= 340 m s^{- 1})$

366 Hz

389 Hz

400 Hz

412 Hz

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

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 10ms−1 ? (Speed of sound in air =340ms−1)