I'm not worried about the trig. I'm just trying to figure out the simplest path to coding the billiard balls effect, and since I just spent $99 on GreenSock, I was hoping to make use of it. So, if I use an "event callback", and write the code to detect ball collision, along with the code to reset ball velocities, is it possible to update the tweened element to a new velocity vector? If not, that means doing my own animation and not using GreenSock at all, in which case I just paid $99 for something I can't use. Then there's the rotation aspect... shoot.