Just a simple physics engine I've been hacking on. It started as just a moving ball to try out html5 canvas, but I decided the ball should bounce. Then it should bounce more realistically (gravity/drag/friction), and in multiple dimensions. Then I decided I might as well do arbitrary vector forces. Then I decided that (since I could easily do thousands of balls), I should do some collision detection (which I should overoptimize with grid-based detection).
Anyway, don't actually use this as either real code or a reference- it's buggy and in a constant state of flux, since it's only for fun.