On a somewhat related note I always thought it would be fun to write alternate physics engines that have consistent rules that aren't realistic but give more cartoonish results such as fast moving objects aren't effected by gravity. Don't have newtons 3rd law but make the more massive or fast moving object be unaffected by what it hits, think of a cartoon character being hit by a cannon ball. The ability to build up momentum in place then release it all at once. It would probably have some huge stability issues in the physics simulation but rules can be added as needed to stabilize without losing the cartoon feel of the engine.