If you need something much more true to the laws of physics, you can look at this demo:

The DEMO FILE is zipped with the required Javascript files (all reside in the same folder).
I have included a number of resource URLs in the JS code, that will be helpful in figuring out how this works.

Just came across this article on the development of the Kinect.  Pretty interesting deep dive into the little piece of hardware that we know has taken on a new life of it's own outside of gaming...