Something I've been doing the last few days. APE has been the first physics engine I've managed to understand and easily use. Box2D confused the crap out of me.
Anyway, click and drag to make objects.
Press and release CONTROL to swap between rectangle and circle.
SPACE to reset.
While APE is easy to use, it is not very stable. Creating a single rectangle will sometimes slide to the right , with nothing but gravity affecting it; nothing stabilises.
s framerate graph [link]
Uses APE (Actionscript Physics Engine) [link]