Box2D is a very easy to use 2D physics library with a growing set of features. Box2D got a lot of attention from the game Crayon Physics that showed the potential of the library. Unfortunately, it is not perfect for the DS, because it uses floating point math, which the DS’s CPUs don’t support. The DS version uses fixed point math.

Release notes:

Here’s a new release of the DS port of Box2D. Robert Kavaler helped me out a lot on this by porting the changes to the SVN version of the (still work-in-progress) Box2D 2.0 and adding a nice build system. Although this is a patch of an unstable version of Box2D it works very stable. By using the DS’s hardware division and sqrt functions I improved simulation speed. Instead of the patches library, the archive now contains a patch with instructions how to acquire and patch Box2D.

http://blog.dev-scene.com/0xtob/