Doppler Effect and Velocity

[aldol16]

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Hi guys, I'm hoping a better physicist than I can answer this question - the Doppler effect tells us that motion of either source or detector will cause a shift in perceived frequency. However, motion of the detector towards the source at some velocity will cause a different shift in frequency as compared to motion of the source towards the detector at the same velocity. In other words, v+vD/v is not the same as v/v-vS. But we also know from physics that velocity is not absolute and depends on frame of reference. In other words, saying that A moves towards B at x m/s is the same as saying that B moves towards A at x m/s. So how do you reconcile these two facts?

Thanks in advance!

 

[NextStepTutor_2]

Hi guys, I'm hoping a better physicist than I can answer this question - the Doppler effect tells us that motion of either source or detector will cause a shift in perceived frequency. However, motion of the detector towards the source at some velocity will cause a different shift in frequency as compared to motion of the source towards the detector at the same velocity. In other words, v+vD/v is not the same as v/v-vS. But we also know from physics that velocity is not absolute and depends on frame of reference. In other words, saying that A moves towards B at x m/s is the same as saying that B moves towards A at x m/s. So how do you reconcile these two facts?

Thanks in advance!

I'm no physicist but I think you may be in error assuming that the Doppler effect is all relative and that one should expect the same result regardless of which body is moving. For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect will result from motion of the source, motion of the observer, or motion of the medium. These effects are each analyzed separately. For waves which do not require a medium, such as light (only other wave type likely to be on the MCAT) only the relative difference in velocity between the observer and the source needs to be considered.

hope this helps, good luck!

 

[aldol16]

Thank you for your explanation!