Doppler Effect

[reising1]

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Question:
Which of the following will result in the greatest increase in the observed frequency of sound?
A. the source moves toward the observer at 30 m/s
B. the observer moves toward the source at 30 m/s
C. the source moves away from the observer at 30 m/s
D. A and B will result in exactly the same frequency

Answer: A

Why is it not D?

 

[shoehornlettuce]

Question:
Which of the following will result in the greatest increase in the observed frequency of sound?
A. the source moves toward the observer at 30 m/s
B. the observer moves toward the source at 30 m/s
C. the source moves away from the observer at 30 m/s
D. A and B will result in exactly the same frequency

Answer: A

Why is it not D?

There are two equations for the doppler effect. One for when the listener is moving toward/away from a stationary source and another where the source is moving toward/away from a stationary listener. The values and sign in the numerator and denominator are different in the two equations resulting in slightly different values.

 

[Addallat]

For A: assuming that the source is stationary the equation would be set up like this

Frequency Observed = Frequency source * Velocity medium + Velocity Observer/Velocity medium

For B: assuming that the observer is stationary the equation would wind up being

Frequency Observed = Frequency Source * Velocity medium / velocity medium - velocity source

 

[reising1]

There are two equations for the doppler effect. One for when the listener is moving toward/away from a stationary source and another where the source is moving toward/away from a stationary listener. The values and sign in the numerator and denominator are different in the two equations resulting in slightly different values.

Do we need to know both equations for the MCAT? I thought all as we needed to know was

wavelength / wavelength_s = frequency / frequency_s = v / c