Now I was reading The Feynman Lectures on Physics and found this which I found somewhat peculiar and deep and thus want your assistance here. So here it goes:
The theorem concerning the motion of the center of mass is very interesting, and has played an important part in the development of our understanding of physics. Suppose we assume that Newton’s law is right for the small component parts of a much larger object. Then this theorem shows that Newton’s law is also correct for the larger object, even if we do not study the details of the object, but only the total force acting on it and its mass. In other words, Newton’s law has the peculiar property that if it is right on a certain small scale, then it will be right on a larger scale. If we do not consider a baseball as a tremendously complex thing, made of myriads of interacting particles, but study only the motion of the center of mass and the external forces on the ball, we find $F=ma$, where $F$ is the external force on the baseball, $m$ is its mass, and a is the acceleration of its center of mass. So $F=ma$ is a law which reproduces itself on a larger scale.
Now here, I do understand that the theorem of center of mass reproduces itself on a larger scale and can figure out why it is so, but I fail to understand how this theorem leads to the conclusion that newton's laws of motion also have this peculiar property.
Other than this, I want to know why Newtonian laws have this replicating property. Is it merely an experimental fact which we have observed and encountered every time we use Newtonian mechanics? Or is there something subtle in the laws themselves which grants them this property of replication on larger scales.
PS: I would request you all to avoid use of concepts of quantum mechanics or something advanced as I'm not in a position to understand that all now. I am only familiar with Newton's laws.
I ask for your help in this regard.