There is SketchY Physics which is a plugin. I don't have the link handy but you'll find it in a Google search. Dynamic components can be made to do some of that, too but you have to have pro to make DCs.
I don't think there's any sort of real collision detection so you have to animate all the gears to make them look like they are turning. some folks are doing some neat things with animated SketchUp stuff but I'm guessing there are easier application for that out there.
For something like doors opening and closing, I think you're just as well off with layers and scenes. With pro you can then export the scenes as an animation and SU interpolates between them.
It works for stuff like this, too. It's a screen grab rather than an AVI export because I wanted the axes to show up.
Irony: The opposite of Wrinkly