Q 11-14-062JEE MainJEE Main 2023 (6 Apr, Shift 1)Medium
A person driving car at a constant speed of $15\ \text{m s}^{-1}$ is approaching a vertical wall. The person notices a change of $40\ \text{Hz}$ in the frequency of his car's horn upon reflection from the wall. The frequency of horn is ______ Hz. (Given: speed of sound $330\ \text{m s}^{-1}$)
Numerical value type. Enter your answer.
Answer: 420
The wall receives $f\dfrac{v}{v-u}$ and reflects it; the driver moving towards the wall hears $f'=f\dfrac{v+u}{v-u}$.
$$f'-f=f\frac{2u}{v-u}=f\times\frac{30}{315}=40\ \Rightarrow\ f=420\ \text{Hz}$$
Solution by Sreeraj P, M.Sc Physics