Thank you very much, nice article, very useful. I love box2d (I've used in c++, java and flash) but I would like to say that nowadays some how when I'm playing online games, or from the apple store, android market, etc... you can easily identify if the app is using box2d or not, absolutely all the games behaves similar (almost the same), which is not too good in terms of uniqueness. Box2d doesn't work with real physics, it does work simulating physics based on Verlet, overlapping objects and recalculating distances and forces on steps, that's why it works so FAST! it's perfect for apps that don't require high accuracy on calculations (real physics are about 10x more expensive in terms of CPU consummation). my advise is, don't expect perfect-physics-simulation from box2d unless you're building simple things, it does the job well, it's good in terms of performance, but also sometimes it looks weird (like in angry birds some massive objects held by tiny objects with reduced mass), hehe! cheers!