Center of gravity

A fixed point on a rigid body through which the resultant force of gravity always passes, regardless of the position of the body.  In a uniform gravitational field, it is identical to the center of mass.  Thus, for practical purposes, the center of gravity and center of mass coincide.  In strict physical terms, however, there is an infinitesimal difference between the two.  The center of gravity represents the unique point in an object used to account for the object’s response to external forces and torques.  For the human body, the center of gravity is not at a fixed anatomical point, but its location varies with the position of the body segments.  The figure below gives an example of how center of mass can be depicted and calculated.

The center of mass is the average of masses relative to their distances from a reference point (O). In one plane, as shown, it is comparable to balancing a seesaw about a pivot point. The formula expresses the center of mass point for a two-mass object.   

See Biomechanics, Center of mass, Force, Gravitational field, Inertia, Moment of inertia (I), Newton’s law of universal gravitation, Postural control, Posture, Torque