Though a little late, I downloaded Box2DFlashAS3 , which is a Flash version of powerful physics library by Erin Catto.
I found that the library is really well-designed and I could made two simple examples like below in a few ten minutes. There still be a functions I want to try like joints that work like gears and pulleys and I'll work on them later.
Only one problem may be the original manual is written in C's grammar. It's not hard to interpret it as AS3 but it's confusing sometimes.