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Question

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__?


A
18/17
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B
19/18
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C
20/19
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D
21/20
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Solution

The correct option is A $$19/18$$
$$f_{app} = f_{0}\left [\dfrac {v_{2} \pm v_{0}}{v_{2}\mp v_{s}}\right ]$$
$$f_{1} = f_{0}\left [\dfrac {340}{340 - 34}\right ]$$
$$f_{2} = f_{0} \left [\dfrac {340}{340 - 17}\right ]$$
$$\dfrac {f_{1}}{f_{2}} = \dfrac {340 - 17}{340 - 34} = \dfrac {323}{306}\Rightarrow \dfrac {f_{1}}{f_{2}} = \dfrac {19}{18}$$.

Physics

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