So, I'm really bothered with something. Let's suppose there's a simple pendulum with a rigid sphere on it's end. In order to get the motion equations I thought we could use two approaches. One would be using rigid body dynamics (torque, moment of inertia ...), the other one would be using Newton's Second Law in the center of mass. The thing is, the results are slightly different depending on the approach. I suppose the second one is wrong, but why is that? Shouldn't the Law be applicable in any system's center of mass? What isn't right with this reasoning?