Two tuning forks `A` and `B` lying on opposite sides of observer 'O' and of natural frequency `80 Hz` move with velocity `10 m//s` relative to stationary observer `O`. Fork `A` moves away from the observer while the fork `B` moves towards him. A wind with a speed `10 m//s` is blowing in the direction of motion of fork `A`. Find the beat frequency measured by the observer in Hz. [Take speed of sound in air as `340 m//s`].

Options

A. 5
B. 6
C. 7
D. 8

Answer

A

Explanation

`f_("observer for source" 'A')=f_(0)[(V_(sound)-V_(medium))/(V_(sound)-V_(medium)-V_(source))]=(33)/(34)f_(0),f_("observer for source" 'A')=f_(0) [(V_(sound)+V_(medium))/(V_(sound)+V_(medium)+V_(source))]=(35)/(34)f_(0)`
`:.` Beat frequency `= f_(1)-f_(2) = ((35-33)/(34))f_(0) = 5`.