Q 11-14-092JEE MainJEE Main 2021 (26 Aug, Shift 1)Medium
A source and a detector move away from each other in absence of wind with a speed of $20$ m s$^{-1}$ with respect to the ground. If the detector detects a frequency of $1800$ Hz of the sound coming from the source, then the original frequency of source considering the speed of sound in the air $340$ m s$^{-1}$ will be ______ Hz.
Numerical value type. Enter your answer.
Answer: 2025
Each moves away from the other at $20$ m s$^{-1}$:
$$f' = f\,\frac{v - v_d}{v + v_s} = f\,\frac{320}{360} = \frac{8f}{9}$$
$1800 = \dfrac{8f}{9} \Rightarrow f = 2025$ Hz.
Solution by Sreeraj P, M.Sc Physics