Q. A motor car blowing a horn of frequency $124vib/sec$ moves with a velocity $72\ km/hr$ towards a tall wall. The frequency of the reflected sound heard by the driver will be (velocity of sound in air is $330\ m/s$)
A) $109\ vib/sec$
B) $132\ vib/sec$
C) $140\ vib/sec$
D) $248\ vib/sec$
Explanation
(c) The frequency of reflected sound heard by the driver
$n^{'} = n\left( \frac{v - \left( - v_{O} \right)}{v - v_{S}} \right) = n\left( \frac{v + v_{O}}{v - v_{S}} \right)$
$= 124\left\lbrack \frac{330 + (72 \times 5/18)}{330 - (72 \times 5/18)} \right\rbrack = 140\ vibration/sec$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.