A stationary source is emitting sound at a fixed frequency `f_(0)`, which is reflected by two cars approaching the source. The difference between the frequencies of sound reflected from the cars is `1.2%` of `f_(0)`. What is the difference in the speeds of the cars (in km per hour) to the nearest integer ? The cars are moving at constant speeds much smaller than the speed of sound which is `330 ms^(-1)`.

Answer

7

Explanation

`f_(app)=f_0.(c+v)/(c-v)`, Where c is speed of sound
`implies df=(2f_0c)/((c-v)^(2))dv approx (2f_0)/(c)dv` …..(i)
Given `df=1.2/100f_0`
From Eqs, (i) and (ii) `dv approx 2m//s=7 km//h`