Gravity, Center of, is the point in an object where the force of gravity appears to act. If an object is suspended from any point on the vertical line passing through its center of gravity, the object will remain stationary. But if the center of gravity is to one side of a point around which the object can pivot, the object will turn in that direction.
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The center of gravity of a child’s seesaw is at the center of the board when no one sits on it. If the seesaw is pivoted at its center, it will remain balanced. When two children of different weights get on opposite ends of the seesaw, the force of gravity will be greater on one end. The center of gravity will then be between the center of the board and the end where the greater force is acting–that is, where the heavier child is sitting. The seesaw will tilt toward that end. If the heavier child moves toward the center of the seesaw, the center of gravity also moves toward the center of the board and it will again balance.
The force of gravity acts on all points in an object where there is mass (material), not simply at the center of gravity. Physicists explain how gravity acts on objects in terms of moments. The moment of the gravitational force acting on any point is found by multiplying the force times the horizontal distance to the pivot point being used. The moment has a size and a direction. That is, the moments of forces acting on one side of a seesaw tend to make it turn up or down, and the moments acting on the opposite side tend to make it turn in the opposite direction. Moments of the same size but opposite direction cancel one another, and so the seesaw does not move.
Thus, in terms of moments, the center of gravity of any object is the point where the moments of the gravitational forces cancel one another. The center of gravity of a seesaw is at the center of the board because the moments cancel one another there.
An object pivoted so that it is not balanced will turn until its center of gravity is at stable equilibrium (the lowest position possible). If the object is balanced with its center of gravity directly above the pivot, the slightest turn will cause the object to become unstable. An object balanced in this way is said to be in a position of unstable equilibrium.
When an object moves, the center of gravity moves as if all the mass of the object were concentrated there. The motion of an object moving under the force of gravity is described as the sum of the motion of the object’s center of gravity and the object’s rotation around its center of gravity. An object’s center of mass is located at the same point as the object’s center of gravity. The center of mass is used to calculate the motion of objects due to all kinds of forces, not only gravitational forces.
