The problem I have is I need to find the force applied to a perfect sphere after colliding with another perfect sphere. For both spheres I have: position as <x,y> Velocity as <x,y> mass coefficient of restitution I am writing a program which simulates a bunch of balls bouncing around so I need to have an series of equations that when using the above values for two balls (that I know are colliding) it gives a force (as <x,y>) that is applied to the current sphere. The force being applied instantaneously. I have searched online and read up on http://en.wikipedia.org/wiki/Inelastic_collision (which only does one dimensional) and other sites like http://www.hoomanr.com/Demos/Elastic2/ . Of the sites that do go into two dimensional they use angle and magnitude, or directly change the velocity of the sphere. I only want the force applied during the collision. I have a decent knowledge of basic physics but for whatever reason I just cannot get this to work properly. What I currently have is a Frankenstein of the wiki page that works relatively well, but it treats every collision as head-on so it isn't ideal.