Modelling "Gravitational" Motion Hi All, I am new here and this is not a homework question... I'm turning 40 this year and my retention of physics is low to say the least. I am trying to rekindle some knowledge... Just for fun! I am trying to use a spreadsheet to model the motion of a system of two particles under gravitational-type forces where one particle has a fixed position and the other is given an initial position and velocity vector. Both particles also have a known mass, e.g., P1(m1,x1,y1) [fixed position] P2(m2,x2,y2,v2) The way I'm approaching this, P2 should move predictably in a time-slice dt. P2 is influenced by its momentum as well as the "gravitational" attraction to P1. I can figure the "gravitational" force vector with no problem. The thing I am hung up on is how to figure the contribution of P2's momentum. I'm not even sure I'm using the correct term here. Once I know how to calculate this vector correctly, I think I can do the math. Please let me know if anything is unclear. Thanks!