A fellow by the name of Chris Cavanagh submitted an XBAP application which demonstrates a simple physics engine, written in C#. You can read more about it here: http://chriscavanagh.wordpress.com/2007/02/21/xbap-2d-physics/ Try it out on this page: http://www.cubpack99.com/pack99/cjc.dynamicsengine.xbap
Note, to move the “vehicle” around, use the Left and Right arrow keys. The Up and Down arrow keys have an effect, too. Reset the whole thing by pressing F5.
Keep in mind, since the WPF Challenge has been changed, you do not have to submit your app as an XBAP. It is perfectly OK to do so, but feel free to submit the source code for a WPF desktop application, too.
Nice work, Chris!