Sound

Question
A man is watching two trains, one leaving and the other coming in with equal speed of 4 m/s. If they sound their whistles, each of frequency 240 Hz, the number of beats heard by the man (velocity of sound in air is 320 m/s) will be equal to
Options:
A. 6
B. 3
C. 0
D. 12

 Next Question
Answer: Option A
: A

Apparent frequency due to train which is coming in is
m1=vvvsn
Apparent frequency due to train which is leaving is
m2=vvvsn
So the number of beats is
n1n2=(13161324)320×240n1n2=6

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