Center of mass

(aka: center of gravity)

The center of mass of an object is the point where the object will always balance. If you could suspend the object by a thread. There would be no tendency for it to tip or turn. Practically speaking, the center of mass and the center of area are located at the same point for flat objects of uniform construction, like a piece of paper or cardboard. You can easily find the center of mass for an unusually shaped piece of cardboard by tying a weight to the end of a string and allowing both the cardboard and the weighted string to hang freely together. Trace the string onto the surface of the cardboard and repeat the procedure allowing the cardboard to hang from a different location. The place where the string markings cross is the center of mass (center of gravity) and, in the case of uniformly constructed flat material (cardboard, sheet plastic, and the like), also the center of area.

 Finding the center of mass of an irregularly shaped sheet of cardboard.