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Questions  

 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 natural frequency of
240 Hz the number of beats heard by the man (velocity of sound in air = 320 m/s) will be equal to

a
6
b
3
c
zero
d
12

detailed solution

Correct option is A

When the train is coming to the observern′=VV−Vsn=320320−4×240 n′′=VV+Vsn=320320+4×240 ≅237Hz When the train is moving away from observer  Beat frequency =n′−n′′=243−237=6

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A sounding body emitting a frequency of 150 Hz is dropped from a height. During its fall under gravity it crosses a balloon moving upwards with a constant velocity of 2 m/s one second after it started to fall. The difference in the frequency observed by the man in balloon just before and just after crossing the body will be: (given that velocity of sound = 300 m/s; g = l0 m/s2)


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