Hi yakas, I've heard of a program called "Working model 2d" which would probably be good for that, but it's pretty expensive: http://www.design-simulation.com/WM2D/index.php

You might also try this: http://physion.net/index.php or there's a plug-in for SketchUp called sketchyphysics which may be able to help.

I'm not familiar with these tools myself though.

- Michael