JEE Advanced · Physics · 16. Waves & Sound
A train moving with a uniform velocity of approaches another train standing on a platform. An observer moves with a uniform velocity of towards as shown in figure. Both the trains are blowing whistles of same frequency When is away from and distance between and is the number of beats heard by is____
[Speed of the sound

- A 8.13
- B 8.14
- C 8.15
- D 8.16
Answer & Solution
Correct Answer
(A) 8.13
Step-by-step Solution
Detailed explanation
Speed of sound=
Calculate beat

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

Where, and are the apparent frequencies of train and 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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