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center of gravity
The point from which the gravitational attraction of a body appears to act. In a uniform gravitational field this is coincident with the body's center of mass. In a distributed mass, the center of gravity (CG) is an appropriately defined "average location" of its parts. If the mass is a rigid body subject to the Earth's gravity, then if it is supported at the CG it will stay balanced and not tilt to any side. In a system subject only to internal forces, the CG always stays in the same spot; hence the Earth-Moon system rotates around its mutual CG (not around the Earth's center), and a rocket flies forward when it ejects a high-speed stream of gas backward. For an expanded body or collection of particles subject to gravitation, the CG is the point through which the resultant force of gravity acts, regardless of the orientation of the body.
The center of gravity of a body of moderate dimensions may be approximately determined by suspending it by a single cord in two different positions, and finding the single body which, in both positions, is intersected by the axis of the cord.
Related categories
• CLASSICAL MECHANICS
• GRAVITATIONAL PHYSICS
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