Perhaps. Of course the addition may give a numerically correct answer - but we then need to consider what is "an inertial frame" ? As gravity cannot be turned off, do we then accept that all inertial frames have zero mass ?
Consider two point mass particles m1 m2 (with m1 different from m2) initially stationary relative to each other a distance x apart . The only force acting on them is attractive Newtonian gravity, i.e. Gm1m2/x^2 acting equally in opposite directions along the line of separation between the...