Question

An observer is standing stationary in air moving at a speed of 2m/s. A source of frequency 20Hz is moving in the direction of wind with speed 10m/s towards the observer. What will be the frequency as heard by the observer? Speed of sound in air is 330m/s.

(a) 20.62Hz

(b) 19.40Hz

(c) 21.68Hz

(d) 22.63Hz

I have been asked this question by my college director while I was bunking the class.

The doubt is from Waves topic in section Waves of Physics – Class 11

Answer 1

Correct option is (a) 20.62Hz

The best explanation: As the air is moving, we can solve this question from the air’s frame of reference.                      From the air’s frame of reference, the observer will be going towards the observer at 2m/s                              and the source will be coming towards the observer at 10-2 = 8m/s.

F = f(v+v0)/(v+v8)

= 20(330+2)/(330-8)

= 20*332/322 = 20.62Hz.