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JEE Main 2021 (24 Feb, Shift 2) · Doppler Effect

Doppler Effect question from JEE Main 2021 (24 Feb, Shift 2)

Q46JEE Main 2021 · 24 FEB · SHIFT 2Medium

Doppler Effect

Two cars are approaching each other at an equal speed of 7.2 km/hr. When they see each other, both blow horns having frequency of 676 Hz. The beat frequency heard by each driver will be ___________ Hz. [Velocity of sound in air is 340 m/s.]

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

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Answer: 8

Speed of each car $= 7.2$ km/h $= 2$ m/s.
Each driver is an observer approaching a source approaching him: $f' = f\frac{v+u}{v-u}$
$f' = 676 \times \frac{342}{338} = 684$ Hz
Beat frequency $= 684 - 676 = 8$ Hz

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