Hello I'm writing a simple 2D ball bouncing simulation with gravity and the ability to bounce off any line segments drawn on the screen. I'm hoping to develop it into a bounce juggling simulator once a get the basic physics right. Here's the exe and source code (Delphi) http://atlas.walagata.com/w/peterbone/ball_bounce.zip . The ball can be thrown by dragging with the left mouse button and surfaces can be drawn by dragging with the right mouse button. I worked out how to do the collision detection and physics myself so I'm not sure it's the best way of doing it but it seems to work well. Here's how it works (skip this paragraph and go to the question at the end if you don't care). It loops through all the surfaces and works out if it's moving towards the surface by doing a dot product with the surface normal and the ball velocity direction. If it's going towards the surface it then works out if it has crossed the surface, since the balls previous position, by using a fast line crossing test (the 2 lines being the surface and the line connecting the ball to its previous position). If it has collided with the surface it calculates the new velocity by reversing the component of the velocity perpendicular to the surface and multiplies it by the restitution. The *position* of the ball is also updated since it has gone beyond the surface. This is done by calculating the intersection and reversing the overshoot in the same way the velocity was updated. Sorry for the long explaination, but does that sound anything like the way it should be done? My main question is, how can I incorporate the spin of the ball into the collision response? I think it must be fairly complex because the spin effects the direction of the ball after the collision and the collision effects the spin. Thanks for any help and sorry if this question has been asked numerous times before. Peter Bone