Q. A person feels 2.5% difference of frequency of a motor-car horn. If the motor-car is moving to the person and the velocity of sound is $320\ m/sec,$ then the velocity of car will be
A) $8\ m/s$ (approx.)
B) $800\ m/s$
C) $7\ m/s$
D) $6m/s$ (approx.)
Explanation
(a) By Doppler’s formula $n^{'} = \frac{nv}{(v - v_{S})}$
Since, source is moving towards the listener so $n^{'} > n.$
If $n = 100$ then $n^{'} = 102.5$
$\Rightarrow 102.5 = \frac{100 \times 320}{(320 - v_{S})} \Rightarrow v_{s} = 8\ m/sec$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.