Q. A train moves towards a stationary observer with speed 34 m/s. The train sounds a whistle and its frequency registered by the observer is $f_1$. If the speed of the train is reduced to 17 m/s, the frequency registered is $f_2$. If speed of sound is 340 m/s, then the ratio $f_1/f_2$ is :

Solution:

$f_{\text{app}} = f_{0} \left[\frac{V_{2} \pm V_{0}}{V_{2} \mp V_{s}}\right] $
$ f_{1} =f_{0} \left[\frac{340}{340-34}\right] $
$f_{2} = f_{0} \left[\frac{340}{340-17}\right] $
$\frac{f_{1}}{f_{2}} = \frac{340 -17}{340-34} = \frac{323}{306} \Rightarrow \frac{f_{1}}{f_{2}} = \frac{19}{18} $

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