Question

A vehicle, with a horn of frequency n is moving with a velocity of 30 m/s in a direction perpendicular to the straight line joining the observer and the vehicle. The observer perceives the sound to have a frequency $(n+n_1)$. If the velocity of sound in air is 300 m/s, then

• A. $n_1=10n$
• B. $n_1=0$
• C. $n_1=0.1n$
• D. $n_1=-0.1n$

Answer 1

The correct option is B $n_1=0$

Here relative motion of vehicle is perpendicular to the observer. So, there is no doppler effect, hence frequency of horn does not changes.
So, frequency at observer is $n$ Hz.
So, $n_1=0$