Realtime Physics simulation in SketchUp : SketchyPhysics

SketchyPhysics (http://sketchyphysics.com) is an Open Source Ruby script for SketchUp (free/pro/Win/Mac) which integrates the Newton Physics engine into SketchUp.
Documentation is scarse, but with some examples you can get started.

You can use it to simulate falling, bouncing and sliding objects, springs, joints... You can make whole mechanisms.

Have fun! Here are some ideas.