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

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

Solution

## The correct option is D $$19/18$$The frequency in first case is:$$f_1 = \left ( \dfrac{340}{340 - 34} \right ) f = \dfrac{10}{9} \,f$$ The frequency in the second case is:$$f_2 = \left ( \dfrac{340}{340 - 17} \right ) f = \dfrac{20}{19}\,f$$ $$\therefore \, \dfrac{f_1}{f_2} = \dfrac{\dfrac{10}{9}}{\dfrac{20}{19}} = \dfrac{19}{18}$$ Physics

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