# Two factories are sounding their sirens at 800 Hz. A man goes from one factory to other at a speed of 2 m/s.The velocity of sound is 320 m/s.The number of beats heard by the person in one second will be :

$(a)\;2\qquad(b)\;4\qquad(c)\;8\qquad(d)\;10$

Sol.. Here sound source is in rest and observer is moving...... So when observer is moving with velocity (Vo) towards source having frequency( n) Then , Apparent frequency Is n’ =n(V+v0)/v. And when observer moves away from source , then apparent freq .n''= n(V-Vo/ v) => N= n’- n'' =) 10 ans. , where, N= No of beats... n= frequency of sound, v= velocity of sound. Vo= speed of observer...(.hope it may b helpful.)