When I search for a physics engine for Google Sketchup I didn't think I would find one.

But I did, and it's free! It's called Sketchyphysics. It allows you to build models, insert engines, motors, wheels, hinges, magnets, etc. Then push play and watch it run. You can even move things while the motion is running.

Unfortunately it is only for Windows right now. They are working on a Mac version but have no release date set yet. They are also working on version 2.

Here are some videos of it in action: