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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 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
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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 $$ 19/18 $$
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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