What is the meaning of center of mass?
In physics, the center of mass of a system of particles is a specific point at which, for many purposes, the system's mass behaves as if it were concentrated. The center of mass is a function only of the positions and masses of the particles that comprise the system. In the case of a rigid body, the position of its center of mass is fixed in relation to the object (but not necessarily in contact with it). In the case of a loose distribution of masses in free space, such as, say, shot from a shotgun, the position of the center of mass is a point in space among them that may not correspond to the position of any individual mass. In the context of an entirely uniform gravitational field, the center of mass is often called the center of gravity - the point where gravity can be said to act. Center of gravity is the sum (or the integral) of the moments of force from the weight of an object divided by the total weight. COG = [w1*r1 + w2*r2 + w3*r3 + ... + wnrn ] / Σwi (i = 1 to n).
COM = [m1*r1 + m2*r2 + m3*r3 + ... + mnrn ] / Σmi (i = 1 to n). In a uniform gravitational field (which is what you usually have), mass is proportional to weight everywhere, so the center of gravity and the center of mass are the same point.