Q. At what speed should a source of sound move so that stationary observer finds the apparent frequency equal to half of the original frequency
A) $v/2$
B) $2v$
C) $v/4$
D) $v$
Explanation
(d) Frequency is decreasing (becomes half), it means source is going away from the observe. In this case frequency observed by the observer is
$n^{'} = n\left( \frac{v}{v + v_{S}} \right) \Rightarrow \frac{n}{2} = n\left( \frac{v}{v + v_{S}} \right) \Rightarrow v_{S} = v$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.