A vehicle sounding a whistle of frequency 256Hz is moving on a straight road,towards a hill with velocity of `10ms^-1`. The number of beats per second observed by a person travelling in the vehicle is (velocity of sound = `300ms^-1`

Options

A. zero
B. 10
C. 14
D. 16

Answer

D

Explanation

Number of beats = Frequency of direct sound - Frequency of reflected sound
`=n'-n`
`n=256, V_s=V_0=10ms^-1`
`implies n'=((V+V_0)/(V-V_S))xxn = ((330+10)/(330-10))xx256`
`=340/320xx 256=272 "Hz"`
`:.` Number of beats=272-256=16