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 particles. Using Newton's second law F= ma, the acceleration of m1 relative to m2 is Gm2/x^2; while the acceleration of m2 relative to m1 is Gm1/x^2. So they have different accelerations, and hence will always have different speeds, relative to each other. So the separation is reducing at 2 different rates for the 2 particles ! But there can only be one speed of (reducing) separation between 2 particles ? Paradox or elementary mistake ?