1. The problem statement, all variables and given/known data Two moving objects (circle shaped) with a known mass, move in a 2D plane with a known constant direction and speed, at certain point, objects collide with eachother (elastic). At that point, the coordinates of the center each object is known, their radius is also known, their Vx and Vy are known (everything whats needed is known, almost). Determine the final speed vector for each object 2. Relevant equations I found this equation: What exactly are the theta and phi angles and how are they calculated on a cartesian plane. Are they calculated in radians or degrees for that equation (I guess that they are in radians, due to the final 1/2 pi). 3. The attempt at a solution I have some troubles determining the angles (theta and phi), because I am not sure whether or not I have to add them certain values (180, 180, 360) depending on the different quadrants( +x+y, -x+y, -x-y, +x-y). Thank you very much!