A man is watching two trains one leaving and other approaching with equal velocities of 4 `ms^(-1)`. If they sound their whistles each of natural frequency 240 hz, the number of beats heard per seceond by the mas will be (velocity of sound in air 320 `ms^(-1)` ) :

Options

A. 6
B. 12
C. 4
D. None of these.

Answer

A

Explanation

here `V_(1) = ((320)/(320 - 4)) 240 = 243` Hz. ,
and `v_(2) = ((320)/(320 + 4)) 240 = 237 Hz. `,
`therefore` beat frequency , b = `v_(1) - v_(2)`
= 243 - 237 = 6 Hz.
Hence the correct choice is (a).