An employee of a factory moving away from his workplace by a car listens to the siren of the factory. He drives the car at the speed of 72 kmh ⁻¹ in the direction of wind which is blowing at 72 kmh ⁻¹ speed. Frequency of siren is 720 Hz. The employee hears an apparent frequency of ____________ Hz. (Assume speed of sound to be 340 ms ⁻¹) Numerical answer type. Work it out, then open the answer.

Answer: 680. Car speed = wind speed = 72 km/h = 20 m/s, both away from the factory. Wind adds to sound speed along its direction: v' = 340 + 20 = 360 m/s Observer moving away: f' = f(v' - u)/(v') = 720 × (360 - 20)/(360) f' = 680 Hz

An employee of a factory moving away from his workplace by a car… | JEE Main 2022 (30 Jun, Shift 1)

Q41JEE Main 2022 · 30 JUN · SHIFT 1Difficult

Doppler Effect

An employee of a factory moving away from his workplace by a car listens to the siren of the factory. He drives the car at the speed of 72 kmh ${}^{-1}$ in the direction of wind which is blowing at 72 kmh ${}^{-1}$ speed. Frequency of siren is 720 Hz. The employee hears an apparent frequency of ____________ Hz.

(Assume speed of sound to be 340 ms ${}^{-1}$ )

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

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Answer: 680

Car speed $=$ wind speed $= 72$ km/h $= 20$ m/s, both away from the factory.
Wind adds to sound speed along its direction: $v' = 340 + 20 = 360$ m/s
Observer moving away: $f' = f\frac{v' - u}{v'} = 720 \times \frac{360 - 20}{360}$
$f' = 680$ Hz

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