Q. A vehicle sounding a whistle of frequency 256 Hz is moving on a straight road, towards a hill with a velocity of$10\ ms^{- 1}$. The number of beats per second observed by a person travelling in the vehicle is (velocity of sound =$330\ ms^{- 1}$)
A) Zero
B) 10
C) 14
D) 16
Explanation
(d) Apparent frequency heard by the observer
$v^{'} = \left( \frac{v + v}{v - v_{s}} \right) \times v$
$= \left( \frac{330 + 10}{330 - 10} \right) \times 256 = \frac{340}{320} \times 256 = 272\ Hz$
$\therefore$No of beats heard by the observer =272-256=16
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.