This was written a few years ago but it seems to be some of the more interesting banter on why using the displacement equation for relative velocity is using acceleration versus constant velocity. It would make sense that since an accelerating bodies displacement would be a curve that you would need calculus to determine the displacement. But computing two object’s relative velocity within a time frame of both of them moving at the same “CONSTANT” velocity away from each other would not required anything more than simple math. IF they are traveling at the same velocity time is the same.
I still believe this all went south when they used a Cartesian coordinate system with the time axis not moving to lay the foundation of the Lorentz Formula dictating that if you travel faster than an object that doesn’t seem to be effected by time you will in effect go back in time.
I really enjoyed the web sites author Joseph A. Rybczyk wording on how he approaches the subject with an open mind as opposed to most of the scientists that seem to just make the math work without questioning the foundation of the question and answers they are supporting.