The diagram shows four situations in which a source of sound S and a detector D are either moving or stationary. The arrows indicate the directions of motion. The speeds are all the same. Detector 3 is stationary. Rank the situations according to the frequency of the source, lowest to highest.
A) 1, 2, 3, 4
B) 4, 3, 2, 1
C) 1, 3, 4, 2
D) 2, 1, 2, 3
E) None of the above

Question

The diagram shows four situations in which a source of sound S and a detector D are either moving or stationary. The arrows indicate the directions of motion. The speeds are all the same. Detector 3 is stationary. Rank the situations according to the frequency of the source, lowest to highest. 
A) 1, 2, 3, 4 
B) 4, 3, 2, 1 
C) 1, 3, 4, 2 
D) 2, 1, 2, 3 
E) None of the above

Quizplus · Accepted Answer

The frequency of sound waves is determined by the speed of the source, the speed of the detector, and the speed of sound. In situations 1 and 2, the source and detector are moving towards each other and away from each other respectively, causing a Doppler shift in frequency. In situations 3 and 4, the source is moving towards/away from the detector while the detector remains stationary. Since the speeds are all the same, the Doppler effect will be the same in situations 1 and 2, resulting in the same frequency. However, since the source is moving directly towards the detector in situation 4, the apparent frequency will be higher due to the compressions of sound waves being closer together. Thus, situation 4 will have the highest frequency and situation 1 will have the lowest frequency. Therefore, option B is correct.

The Diagram Shows Four Situations in Which a Source of Sound