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

Apparent frequency due to train which is coming in is
`n_1=(v)/(v-v_s)n`
Apparent frequency due to train which is leaving is
`n_2=(v)/(v+v_s)n`
So the number of beats is
`n_1-n_2=((1)/(316)-(1)/(324))320xx240impliesn_1-n_2=6`