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JEE Main 2021 (27 Aug, Shift 1) · Doppler Effect

Doppler Effect question from JEE Main 2021 (27 Aug, Shift 1)

Q43JEE Main 2021 · 27 AUG · SHIFT 1Easy

Doppler Effect

Two cars X and Y are approaching each other with velocities 36 km/h and 72 km/h respectively. The frequency of a whistle sound as emitted by a passenger in car X, heard by the passenger in car Y is 1320 Hz. If the velocity of sound in air is 340 m/s, the actual frequency of the whistle sound produced is .................. Hz.

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

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Answer: 1210

$v_X = 36$ km/h $= 10$ m/s (source), $v_Y = 72$ km/h $= 20$ m/s (observer), approaching.
$n' = n\frac{v + v_o}{v - v_s} \Rightarrow 1320 = n \times \frac{340 + 20}{340 - 10} = n \times \frac{360}{330}$
$n = 1320 \times \frac{330}{360} = 1210$ Hz

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