Libraries

libgdx comes with 2 different Physics libraries. Box2D which is used for 2D physics and also a experimental Bullet Physics wrapper which can be used for 3D physics.

Documentation

Box2D Documentation - Most documentation is compatible with the libgdx implementation but will need some changes from C++ to Java.

Bullet Wiki - Documentation for Bullet should be compatible with the Java wrapper of Bullet but will need various syntax and class name changes.