Where do I start to write/use a 3D physics simulation engine?

Question

I need to write a very simple 3D physics simulator in Java, cube and spheres bumping into each other, not much more. I've never did anything like that, where should I start? Any documentation on how it is done? any libraries I could re-use?

Answer 1

Physics for Game Programmers By Grant Palmer (not Java)

Phys2D (Java code)