Physics in Flash
I’ve been most impressed with the collision detection and response (i.e. “physics”) tutorials provided by Metanet, creators of N. They explain some fairly robust 2D collision detection and response in ActionScript, in particular their grid-based and raycasting solutions. Definitely check out their slides from their FiTC 2005 presentation.
There’s also the Flash Dynamics Engine, which uses some proper physics algorithms (Verlet integration) to do a more general physics solver, though it’s more expensive (but more realistic) than Metanet’s stuff above.