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 `320m//s` will be equal to

Options

A. 6
B. 3
C. 0
D. 12

Answer

A

Explanation

`f_(max)=f((v)/(v-v_(S)))=240((320)/(320-4))=243Hz`
`f_(min)=f((v)/(v+v_(S)))=240((320)/(320+4))=237Hz`
`f_("beat")=243-237=6Hz`.