http://www.ibm.com/developerworks/opensource/library/os-physicsengines/index.html
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Open source physics engines
Building believable worlds with open source
M. Tim Jones, Consultant Engineer, Independent author
Summary: Graphics give games a visual appeal, but it's the internal physics engine that gives the game's world life. A physics engine is a software component that provides a simulation of a physical system. This simulation can include soft- and rigid-body dynamics, fluid dynamics, and collision detection. The open source community has a number of useful physics engines operating in the 2D and 3D domains targeted to games and simulations. This article introduces the use and basics of a physics engine and explores two options that exist: Box2D and Bullet.
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http://chrishecker.com/Rigid_Body_Dynamics