Cyclone

Cyclone is a very simple physics simualtion system written for the book Games Physics Engine Development Ian Millington, Morgan Kaufmann Publishers, 2007. The code was designed to allow me to step through the creation of a physics engine in the text. It is aimed at people who want an introduction to how physics works, rather than people who want to use physics in their game. For the latter, you'd be much better off with a library like Havok, PhysX, bullet, or even ODE (although that is showing its age somewhat).

The book's website is available here.