Just a head's up if you decide to go forward with using SketchyPhysics...it's on the Quarantine list because it interferes and modifies some things about SketchUp.
http://sketchucation.com/forums/viewtopic.php?f=323&t=47371&p=456723&hilit=quarantine#p456723
You can of course still install and use it, just know that you might get some funky behavior and busted tools/behavior. I didn't know this when I first used it, and it gave me a lot of headaches.