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 $\frac{f_{1}}{f_{2}}$ is

18/19

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1/2

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19/18

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Solution

The correct option is D 19/18
By using $n^{'} = n (\frac{v}{v - v_{s}})$

$\Rightarrow f_{1} = n (\frac{v}{v - v_{s}}) = n (\frac{340}{340 - 34}) = \frac{340}{306} n$

and $f_{2} = n (\frac{340}{340 - 17}) = n (\frac{340}{323}) \Rightarrow \frac{f_{1}}{f_{2}} = \frac{323}{306} = \frac{19}{18}$

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A train moves towards a stationary observer with speed 34 m/s. The train sounds a whistle and its frequency registered is f1. 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 $\frac{f_{1}}{f_{2}}$ is

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 $\frac{f_{1}}{f_{2}}$ is :