Q. A person carrying a whistle emitting continuously a note of $272\ Hz$ is running towards a reflecting surface with a speed of $18\ km/hour.$ The speed of sound in air is $345ms^{- 1}.$ The number of beats heard by him is
A) 4
B) 6
C) 8
D) 3
Explanation
(c) According the concept of sound image
$n^{'} = \frac{v + v_{person}}{v - v_{person}}.272 = \frac{345 + 5}{345 - 5} \times 272 = 280\ Hz$
$\mathrm{\Delta}n =$ Number of beats $= 280 - 272 = 8\ Hz$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.