Originally Posted by
A Squared
Look, it's pretty clear that you simply are unable to grasp the concept of Velocity, how it is different than "speed", why it is imperative that velocity is used in physics and not speed, and why a constant, unchanging frame of reference is necessary to make any kind of a meaningful analysis. I apologize for having wasted your time trying to explain these things and I will stop now.
Quit being a martyr. You know it's perfectly reasonable, given the standard set of assumptions, to say that your groundspeed is 90 knots one way, then 110 the other way. In fact, before this thread, you would not have even blinked at saying exactly that. You know it.