A train approaching a railway platform with a speed of 20 m s-1 starts blowing the whistle. Speed of sound in air is 340 m s-1. If the frequency of the emitted sound from the whistle is 640 Hz, the frequency of sound as heard by person standing on the platform is

Question

A train approaching a railway platform with a speed of 20 m s-1 starts blowing the whistle. Speed of sound in air is 340 m s-1. If the frequency of the emitted sound from the whistle is 640 Hz, the frequency of sound as heard by person standing on the platform is (A) 600 Hz (B) 640 Hz (C) 680 Hz (D) 720 Hz

Tardigrade Admin · Accepted Answer

Given: v=340 ms -1, us=20 ms -1 and v0=640 Hz From Doppler's law, v=((340/340-20)) 640 v=680 Hz

Q. A train approaching a railway platform with a speed of $20 m s^{- 1}$ starts blowing the whistle. Speed of sound in air is $340 m s^{- 1}$ . If the frequency of the emitted sound from the whistle is $640 Hz$ , the frequency of sound as heard by person standing on the platform is

$600 Hz$

$640 Hz$

$680 Hz$

$720 Hz$

Given : $v = 340 m s^{- 1}, u_{s} = 20 m s^{- 1}$ and
$v_{0} = 640 Hz$
From Doppler's law,
$v = (\frac{340}{340 - 20}) 640$
$v = 680 Hz$

Q. A train approaching a railway platform with a speed of 20ms−1 starts blowing the whistle. Speed of sound in air is 340ms−1. If the frequency of the emitted sound from the whistle is 640Hz, the frequency of sound as heard by person standing on the platform is

600Hz

640Hz

680Hz

720Hz