Center of Mass in Physics

A thrown hammer tumbles end over end, and yet one point inside it traces a plain parabola. That point is the center of mass, and it is the reason a complicated extended object can still be treated as a single moving body. It is a mass weighted average of position, so heavier parts pull it towards themselves, and it obeys Newton's second law on its own with the total mass and the total external force. The blocks give the averaging rules for position, velocity and acceleration, the shortcut for two bodies, and the law of motion the whole system obeys.