I want to clarify something basic, since physics is not really my field of expertise. If an observer is staying in an Inertial Reference Frame, and the other observer is inside a ship already having some relativistic velocity ( relative to the first observer ) and currently accelerating. Would the two observer observe the same value of the ship's acceleration ? The link below is my attempt to do the math. Hope someone have the time to review it. http://orimath.blogspot.com/2008/05/mathematical-basis-of-relativistic.html Thank you.