planar constraint

[kangaroo]

Hi. Though it is not directly related to bullet, can somebody help me out a little bit. Lets take take a particle bouncing off a plane. From an impulse based point of view, constraint equation would look something like this (Vn+ + e*Vn-)/(1+e)=0, where e - coefficient of restitution, Vn+ - normal component of post-impact velocities, Vn- - pre-impact. Can this expression be written from a force-based point of view?