Question
An observer at rest is standing near the train which blows the signal of frequency 400 Hz. Another train is approaching the observer with velocity 5 m/sec which also blows the signal with frequency 400 Hz. If the observer hears 6 beats in one second then what is the velocity of sound?
Solution
The apparent frequency of signal from train approaching towards observer is given by,
...(1)
where,
v is the velocity of sound and u is the velocity of the source.
The actual frequencies of the signal from both the sources are same.
Therefore, number of beats heard by the observer is,
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Here, we have
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, 
Therefore,
Velocity of the sound is,
...(1) where,
v is the velocity of sound and u is the velocity of the source.
The actual frequencies of the signal from both the sources are same.
Therefore, number of beats heard by the observer is,
Here, we have
, Therefore,
Velocity of the sound is,
