Question

A vehicle, with a horn of frequency n is moving with a velocity of 30m/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+n1. Then (if the sound velocity in air is 300m/s)

(a) n1=0.1n

(b) n1=0

(c) n1=10n

(d) n1=-0.1n

Answer 1

Correct choice is (b) n1=0

To elaborate: As the vehicle moves in a direction perpendicular to the line joining the observer and the vehicle, the component of the velocity along this direction zero. There is no Doppler effect and the frequency of sound remains unchanged.