A source of sound of frequency 300 Hz and a receiver are located along the same line normal to the wall as shown in the figure. Both the source and the receiver are stationary and the wall receeds from the source with velocity `20ms^-1`.If the beat frequency registered by the receiver is `240/x` Hz then x is :
(Assume `V_("sound")=300` m/s).

Explanation

As , `f_1=f` (For direct sound)
Now, for reflected sound `f_2=((V-20)/(V+20))f`
If b is the beat frequency :
`therefore b=f_1-f_2`
`therefore f-((V-20)/(V+20))f =(f.40)/(v+20)`
=`(300.40)/350=240/7` Hz