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JEE Main 2021 (20 Jul, Shift 1) · Doppler Effect

Doppler Effect question from JEE Main 2021 (20 Jul, Shift 1)

Q45JEE Main 2021 · 20 JUL · SHIFT 1Medium

Doppler Effect

The frequency of a car horn encountered a change from 400 Hz to 500 Hz, when the car approaches a vertical wall. If the speed of sound is 330 m/s. Then the speed of car is ___________ km/h.

Numerical answer type. Work it out, then open the answer.

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Answer: 132

Echo: $\dfrac{500}{400} = \dfrac{330 + u}{330 - u}$
$5(330 - u) = 4(330 + u) \Rightarrow 9u = 330 \Rightarrow u = 36.67\text{ m/s}$
$u = 36.67 \times 3.6 = 132\text{ km/h}$

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