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JEE Main 2019 (10 Jan, Shift 1) · Doppler Effect

Doppler Effect question from JEE Main 2019 (10 Jan, Shift 1)

Q54JEE Main 2019 · 10 JAN · SHIFT 1Medium

Doppler Effect

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 ƒ1. If the speed of the train is reduced to 17 m/s, the frequency registered is ƒ2. If speed of sound is 340 m/s, then the ratio ƒ1/ƒ2 is -

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Answer: (a)

Source approaching: $f' = f\,\dfrac{v}{v - u}$ , so $\dfrac{f_1}{f_2} = \dfrac{v - u_2}{v - u_1}$ .
$\dfrac{f_1}{f_2} = \dfrac{340 - 17}{340 - 34} = \dfrac{323}{306} = \dfrac{19}{18}$
So the answer is (a) 19/18.

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