I've read that if the sum of the forces on a rigid body is zero, then the center of mass will not accelerate(the body may rotate around the center of mass even though the sum of the forces is zero), and that Newtons second law $F=ma$ holds for the center of mass. But is this proven from Newtons second law used on all the small masses making up the rigid body?, or is this a law that has only been shown to be correct experimentally?
If this version of Newtons second law can be proven by the simpler form on particle masses, how do we show it?