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JEE Main 2020 (5 Sep, Shift 2) · Doppler Effect

Doppler Effect question from JEE Main 2020 (5 Sep, Shift 2)

Q48JEE Main 2020 · 5 SEP · SHIFT 2Medium

Doppler Effect

A driver in a car, approaching a vertical wall notices that the frequency of his car horn, has changed from 440 Hz to 480 Hz, when it gets reflected from the wall. If the speed of sound in air is 345 m/s, then the speed of the car is:

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Answer: (b)

Echo: $\dfrac{480}{440} = \dfrac{12}{11} = \dfrac{345 + u}{345 - u}$
$12(345 - u) = 11(345 + u) \Rightarrow 23u = 345 \Rightarrow u = 15\text{ m/s}$
$15 \times 3.6 = 54\text{ km/h}$
So the answer is (b) 54 km/hr.

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