Farseer Physics Engine 是个好东西，虽然比较简单，但功能完整。
正在学习和分析这个简单例子
Farseer 物理引擎的主要功能有：
* Continuous collision detection (with time of impact solver)
* Contact callbacks: begin, end, pre-solve, post-solve
* Convex polyons and circles.
* Multiple shapes per body
* Dynamic tree broadphase
* Fast broadphase AABB queries
* Collision groups and categories
* Sleep management
* Contact, friction, and restitution
* Stable stacking with a linear-time solver
* Revolute, prismatic, distance, pulley, gear, mouse joint, and other joint types
* Joint limits, motors, and friction
* Controllers (gravity, force generators)
* Tools to decompose concave polygons, find convex hulls, boolean operations and more.