A person driving car at a constant speed of 15 m/s is approaching a… | JEE Main 2023 (6 Apr, Shift 1)

Q37JEE Main 2023 · 6 APR · SHIFT 1Medium

Doppler Effect

A person driving car at a constant speed of $15\text{ m/s}$ is approaching a vertical wall. The person notices a change of $40\text{ Hz}$ in the frequency of his car's horn upon reflection from the wall. The frequency of horn is _______________ $\text{Hz}$ .

(Given: Speed of sound: $330\text{ m/s}$ )

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

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Answer: 420

Echo frequency $f' = f\,\dfrac{v + u}{v - u} = f\,\dfrac{345}{315}$ .
Change $f' - f = f\,\dfrac{30}{315} = 40$
$f = \dfrac{40 \times 315}{30} = 420\text{ Hz}$

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