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NEET 1991 (AIPMT) · Doppler Effect

Doppler Effect question from NEET 1991 (AIPMT)

Q83NEET 1991 · AIPMTMedium

Doppler Effect

Two trains move towards each other with the same speed. The speed of sound is $340\text{ m/s}$ . If the height of the tone of the whistle of one of them heard on the other changes to $9/8$ times, then the speed of each train should be

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Answer: (a)

Both approaching with speed $u$ : $\dfrac{f'}{f} = \dfrac{340 + u}{340 - u} = \dfrac{9}{8}$ .
$2720 + 8u = 3060 - 9u \Rightarrow 17u = 340$
So the answer is (a) $u = 20\text{ m/s}$ .

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