Two submarines are moving towards each other. The first submarine emits a wave towards the second. The wave bounces off the second submarine and reaches the first submarine again.
How should one calculate the frequency of the reflected wave?
The main question here is what speeds should we consider for the reflection when calculating the Doppler effect. From what I understand, if the second submarine is moving when the reflection is happening, so both the observer and the source are still moving towards each other, hence:
f' = f * [ (v-o) / (v-s) ]
Where o is the speed of the observer (first sub) and s is the speed of the source (second sub).
But here's the thing, when discussing this in class, my physics teacher mentioned that the speed of the source (second sub) should be zero, because we already considered its speed when calculating the frequency of the wave before the reflection, in that case the speed of the observer.
And I really couldn't understand this.
Why shouldn't I consider anymore the speed of the source when reflecting a Doppler affected wave?