Before the weekend, I suggest to dig into the Box2D API for Flash with the great WorldConstructionKit.
Box2D is a physical engine wrote in C++ and translated into many languages such as Java, Objective-C, AS3, JavaScript…
For ActionScript3, there are 2 ports : Boris the brave’s port, and the WCK. You can find a performance comparison here made by Allan Bishop.
The WCK is the best thanks to the Alchemy port : it translates the C++ into something which can be understable by Flash. More information here.
The WCK provides a “a toolset / framework for rapidly developing physics based games / websites within the Flash IDE. WCK allows you to layout your 2d worlds / game levels entirely within Flash and have them hook into the physics simulation without writing any code.”