Pedro Lourenço and Bilou have been working on a project that involves using the APE physics engine for Flash. Here's a demo of using APE that allows to play a bit with different parameters of the engine like damping, gravity, mass, etc. The source code has a lot of comments ( as does the engine itself ) that should permit anyone starting with APE to get going.

Some minor issues/glitches on the engine are noticeable, mainly when you crank up strength values, or at least make masses way to small for the forces being applied to them. You can spot them when the ball is re spawned near the screen centre, this probably happens because objects start moving so fast that once the engine tracks where they're at, faults have already occurred. Overlapping also occurs, a bit more often than it should but there are some ways to workaround it...or you can learn to live with it!

Go ahead, try the demo to understand APE a bit better, "a" and "s" move the pong around, "r" resets the level once all targets are gone. Fiddle with the values on the bottom and press enter to set them.

Source Code (tested in Flex Builder 3)