Question

In the diagram below, a stationary source located at point S produces sound having a constant frequency of 512 hertz. Observer A, 50 meters to the left of S, hears a frequency of 512 hertz. Observer B, 100.0 meters to the right of S, hears a frequency lower than 512 hertz.
A) Observer A is stationary, and observer B is moving away from point S.

B) Observer A is stationary, and observer B is moving toward point S.

C) Observer A is moving away from point S, and observer B is stationary.

D) Observer A is moving toward point S, and observer B is stationary.

Answer 1

This has to do with Doppler effect. If the source or the observer moves, they the observer will hear a different frequency. Since the source is stationary, we only need to know whether the observer is moving.

Since Observer A does not hear a change in frequency, he a must be stationary. So we can ignore C and D

Since Observer B hears a different frequency, Observer B must be moving. Here's the tricky part. We need to find whether B is moving towards or away the source. Since he hears a lower frequency, he is walking away from the source. So the answer is A.

You can conform with the Doppler equation.

Answer: A