In a simple flat 2 dimensional plane of a two body orbit, both circular & elliptic. How do you write the functions wrt “t” time for: 1) Force vector equation (that applies acceleration) 2) Velocity vector equation 3) Position vector equation That allows applying simple calculus integration to obtain #3 from #2 and #2 from #1 Likewise simple calculus should show #1 as derivative of #2, and #2 as derivative of #3. Are polar coordinates preferred? or can X&Y coordinates work as well. If two masses are both large, (from equal to one 10% of other) is it necessary to use a barycentre with a “reduced mass”?