Graphics: Graphics Programs

Graphics Programs

I'm looking for a free 3D modeling program that has simulated physics.

By that I mean that after i design the shapes I can tell it that the shapes connect and how, And have it model the physics of what I want it to do. To let me know if I did it right.


(an example)

I make two wheels and attach them to a pole on a hill, if I modeled the wheels and such correctly it rolls down the hill.


So does any one know of any program that will do this?(prefer a free one but if there aren't any I would appreciate any info on one.)