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JEE Main 2019 (12 Apr, Shift 2) · Doppler Effect

Doppler Effect question from JEE Main 2019 (12 Apr, Shift 2)

Q58JEE Main 2019 · 12 APR · SHIFT 2Medium

Doppler Effect

Two sources of sound S1 and S2 produce sound waves of same frequency 660 Hz. A listener is moving from source S1 towards S2 with a constant speed u m/s and he hears 10 beats/s. The velocity of sound is 330 m/s. Then, u equals:

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Answer: (d)

  1. Moving away from S1S_1: f1=fv−uvf_1 = f\frac{v-u}{v}; moving towards S2S_2: f2=fv+uvf_2 = f\frac{v+u}{v}
  2. Beats =f2−f1=2fuv= f_2 - f_1 = \frac{2fu}{v}
  3. 10=2×660×u330=4u⇒u=2.510 = \frac{2 \times 660 \times u}{330} = 4u \Rightarrow u = 2.5 m/s
  4. So the answer is (d) 2.5 m/s.
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