If the mass moment with respect to a particular axis is numerically zero, then that
axis passes through the center of mass (CM) of the object, which for earthbound systems
is coincident with its center of gravity (CG). By definition the summation of first
moments about all axes through the center of gravity is zero. We will need to locate the
CG of all moving bodies in our designs because the linear acceleration component of
each body is calculated acting at that point.