First slide

Doppler's effect

Question

An observer standing at station observes frequency 219 Hz when a train approaches and 184 Hz when train goes away from him. If velocity of sound in air is 340 m/s, then velocity of train and actual frequency of whistle will be

Moderate

A

$15 .5 {ms}^{- 1}, 200 Hz$
B

$19 .5 {ms}^{- 1}, 205 Hz$
C

$29 .5 {ms}^{- 1}, 200 Hz$
D

$32 .5 {ms}^{- 1}, 205 Hz$

Solution

When train is approaching frequency heard by the observer is
$n_{a} = n (\frac{v}{v - v_{S}}) \Rightarrow 219 = n (\frac{340}{340 - v_{S}})$ …(i)
when train is receding (goes away), frequency heard by the observer is
$n_{r} = n (\frac{v}{v + v_{s}}) \Rightarrow 184 = n (\frac{340}{340 + v_{s}})$ …(ii)
On solving equation (i) and (ii) we get $n = 200 Hz$
and $v_{S} = 29 .5 m / s .$

Get Instant Solutions

When in doubt download our app. Now available Google Play Store- Doubts App

Recieve an sms with download link

Doubts App

OR

SCAN ME

Doubts App

Doubts App