Really erks me, flash designer with an iPhone, tricking blows I can't view my own animations with out converting them. Anyway, box2d is an easy library to work with in flash. Adobe created a program that converts the source code of a library (I think it supports C++, hAxE and someother can't remember.) to AS3. Box2D has a component based physics engine for the FlashIDE. This means, you can create a shape, define it as a component, pick it's properties, and it will automatically obtain physic attributes. You can also hinge oobjects together making it easy to create cars and such.
The game Fantastic Contraption uses Box2D, allowing for the very flexible gameplay experience.