APE Users Guide is a book designed to help you get the most out of the open source 2D physics engine APE.
About
APE (Actionscript Physics Engine) is written in AS3 and is designed for use in Flash or Flex games and applications.
Overview

APE is a 2D physics engine written in Actionscript 3 for use in Flash and Flex.
You can simulate 2D dynamics by using particles and connecting them with constraints to create more complex shapes. The engine currently has 3 different kinds of particle classes - the CircleParticle, RectangleParticle, and WheelParticle, and one type of constraint - the SpringConstraint.
Setup
Flex Builder 2
Flash CS3
A good tutorial here (this is Part 2, which directly addresses Flash CS3, as opposed to part one which works with Flex): http://www.emanueleferonato.com/2007/10/23/ape-actionscript-physics-engine-tutorial-part-2/
Particles
CircleParticle
RectangleParticle
WheelParticle
Constraints
SpringConstraint
Grouping
Group
Composite
Collisions
Static Collision Tests
Multisampling
Painting and Rendering
Built-in methods
Custom methods and overriding
License
APE is released under the open source MIT license.