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  4. 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 f1. If the train's speed is reduced to 17 m/s, the frequency registered is f2. If the speed of sound is 340 m/s, then the ratio f1/ f2 is

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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 train's speed is reduced to 17 m/s, the frequency registered is $f_2$. If the speed of sound is 340 m/s, then the ratio $f_1/ f_2$ is

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Solution:

$ f_1=f \bigg(\frac{v}{v-v_s}\bigg)$
$ f_1=f \bigg(\frac{340}{340-34}\bigg)= f\bigg(\frac{340}{306}\bigg)$
and $ f_2=f \bigg(\frac{340}{340-17}\bigg)= f\bigg(\frac{340}{323}\bigg)$
$\therefore \frac{f_1}{f_2}=\frac{323}{306}= \frac{19}{18}$