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Question

A train moves towards a stationary observer with speed $$34\ m/s$$. The train sound a whistle and its frequency registered by the observer is $${f}_{1}$$. If the trains speed is reduced to $$17\ m/s$$, the frequency registered is $${f}_{2}$$. If the speed is reduced to $$340\ m/s$$ then ratio $${f}_{1}/{f}_{2}$$ is


A
18/19
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B
1/2
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C
2
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D
19/18
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Solution

The correct option is D $$19/18$$
for stationary observes 
$$f=\dfrac{v}{v-u} \times$$ Original frequency 
Situation $$1$$.
$$f_{1} =\dfrac{340}{340-34} f$$

     $$=\dfrac{340}{306} f$$
Situation $$2$$.
$$f_{2}= \dfrac{340}{340-17}$$

     $$=\dfrac{340}{323}$$

Now $$\dfrac{f_{1}}{f_{2}}= \dfrac{340}{306} \times \dfrac{323}{340}$$

        $$\dfrac{f_{1}}{f_{2}}=\dfrac{19}{18}$$

Physics

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