A man standing on a platform observes that the frequency of the sound of a whistle emitted by a train drops by 140 Hz. If the velocity of sound in air is 330 m / s and the speed of the train is 70 m / s, the frequency of the whistle is

Question

A man standing on a platform observes that the frequency of the sound of a whistle emitted by a train drops by 140 Hz. If the velocity of sound in air is 330 m / s and the speed of the train is 70 m / s, the frequency of the whistle is (A) 571 Hz (B) 800 Hz (C) 400 Hz (D) 260 Hz

Tardigrade Admin · Accepted Answer

Velocity of sound in air =330 m / s. Speed of train =70 m / s Let initial frequency be f0 f=f0 × (v/v+vs) f0-f=140 f0(1-(330/400))=140 f0=800 Hz

Q. A man standing on a platform observes that the frequency of the sound of a whistle emitted by a train drops by $140 Hz$ . If the velocity of sound in air is $330 m / s$ and the speed of the train is $70 m / s$ , the frequency of the whistle is

571 Hz

800 Hz

400 Hz

260 Hz

Velocity of sound in air $= 330 m / s$ .
Speed of train $= 70 m / s$
Let initial frequency be $f_{0}$
$f = f_{0} \times \frac{v}{v + v _{s}}$
$f_{0} - f = 140$
$f_{0} (1 - \frac{330}{400}) = 140$
$f_{0} = 800 Hz$

Q. A man standing on a platform observes that the frequency of the sound of a whistle emitted by a train drops by 140Hz. If the velocity of sound in air is 330m/s and the speed of the train is 70m/s, the frequency of the whistle is