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JEE Advanced · Physics · 16. Waves & Sound

A train S1, moving with a uniform velocity of 108 km/h, approaches another train S2 standing on a platform. An observer O moves with a uniform velocity of 36 km/h towards S2, as shown in figure. Both the trains are blowing whistles of same frequency 120 Hz. When O is 600 m away from S2 and distance between S1 and S2 is 800 m, the number of beats heard by O is____
[Speed of the sound =330 m/s]

  1. A 8.13
  2. B 8.14
  3. C 8.15
  4. D 8.16
Verified Solution

Answer & Solution

Correct Answer

(A) 8.13

Step-by-step Solution

Detailed explanation

Speed of sound= 300ms
Calculate beat freq

fb=f1-f2
Where, f1 and f2 are the apparent frequencies of train S1 and S2 observed by observer.
Using Doppler's effect,
\(\Rightarrow f_b=120\left[\left(\frac{330+10 \cos 53^{\circ}}{330-30 \cos 37^{\circ}}\right)-\left(\frac{330+10}{330}\right)\right]\) \(=120\left[\frac{336}{306}-\frac{34}{33}\right]=8.128 Hz\)
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