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

Answer

A

Explanation

Frequency of sound heard by the man from approaching train
`n_a=n(v/(v-v_s))=240(320/(320-4))=243Hz`
Frequency of sound heard by man from receding train
`n_r=n(v/(v+v_s))=240(320/(320+4))=237Hz`
Hence, number of beats heard by man per sec
`=n_a-n_r=243-237=6`