Question

A vehicle with horn of frequency $ \upsilon $ is moving with a velocity of $30\, m/s$ in a direction perpendicular to the line joining the observer and the vehicle. The observer perceives the sound to have a frequency $ (v+{{v}_{1}}) $ . If the sound velocity in air is $300 \,m/s$, then.

• A. $ {{v}_{1}}=0 $
• B. $ {{v}_{1}}=0.1\,v $
• C. $ {{v}_{1}}=-0.1\,v $
• D. $ {{v}_{1}}=10\,v $

Answer 1

Answer: (A)

Given : Actual frequency of horn $=v$ Apparent frequency perceived by observer
$v=v+v_{1}$
Velocity of vehicle $v_{s}=30 \,m / s$
Velocity of sound in air $v=300\, m / s$
Doppler's effect of sound is valid only if source and observer are both in relative motion.
As the source is moving in a direction perpendicular to the straight line which joins the observer hence, there is no relative motion between them.
Therefore, Doppler's effect will not occur.
Thus, $v_1 = 0$