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JEE Main 2020 (4 Sep, Shift 2) · Doppler Effect

Doppler Effect question from JEE Main 2020 (4 Sep, Shift 2)

Q49JEE Main 2020 · 4 SEP · SHIFT 2Medium

Doppler Effect

The driver of a bus approaching a big wall notices that the frequency of his bus's horn changes from 420 Hz to 490 Hz when he hears it after it gets reflected from the wall. Find the speed of the bus if speed of the sound is 330 ms–1.

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Answer: (a)

Echo: $\dfrac{490}{420} = \dfrac{7}{6} = \dfrac{330 + u}{330 - u}$
$7(330 - u) = 6(330 + u) \Rightarrow 13u = 330 \Rightarrow u \approx 25.4\text{ m/s}$
$25.4 \times 3.6 \approx 91\text{ km/h}$
So the answer is (a) 91 km/h.

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