Q. A whistle revolves in a circle with an angular speed of $20\ rad/sec$ using a string of length $50\ cm.$ If the frequency of sound from the whistle is $385\ Hz,$ then what is the minimum frequency heard by an observer, which is far away from the centre in the same plane? $(v = 340\ m/s)$
A) $333\ Hz$
B) $374\ Hz$
C) $385\ Hz$
D) $394\ Hz$
Explanation
(b) Minimum frequency will be heard, when whistle moves away from the listener.
$n_{\min} = n\left( \frac{v}{v + v_{s}} \right)$ where $v = r\omega = 0.5 \times 20 = 10\ m/s$
$\Rightarrow n_{\min} = 385\left( \frac{340}{340 + 10} \right) = 374Hz$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.