The center of mass is the balance point of a system, a point where you could consider the mass of the whole system to be concentrated. In a system which has moveable joints, i.e. the system can move or be moved into different configurations or positions, the location of the center of mass depends on the position of the system. For example, when a figure skater lifts his or her arms during a spin, the mass of the arms is higher up in the system than when the arms were at the side of the skater. With more mass of the skater distributed in a higher position in the body, the center of mass of the skater rises to a higher position in the body. This concept is illustrated in the following figure.
© April, 1998, Montana State University-Bozeman