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Question

A train moves towards a stationary observer with a speed 34 ms$$^{-1}$$. The train sounds a whistle and its frequency registered by the observer is f$$_{1}$$. If the trains speed is reduced to 17 ms$$^{-1}$$, then the frequency registered is f$$_{2}$$. If the speed of the sound is 340 ms$$^{-1}$$, then the 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
A  train moves towards a stationary observer with a speed $$V_s\ m/s$$. The train sounds a whistle and its frequency registered by the observer is $$f_1=n \dfrac {V}{V-V_s}$$. So for two different velocity the ratio of apparent frequencies will be:

$$f_1/f_2=\dfrac {V-V_{s2}}{V-V_{s1}}$$
Putting corresponding values in above equation will get,
$$f_1/f_2=\dfrac {340-17}{340-34}=\dfrac{19}{18}$$.

Physics

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