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A seesaw works by balancing moments.

A moment is a measurement of rotational tendency around a point, expressed by the equation:

moment = force × perpendicular distance

The moment required to rotate a given object around a given point is constant, regardless of where the force is applied. A door, for example, may be opened by applying the necessary force a given distance from the hinge, or by applying twice the force at one-half the distance, or four times the force at one-fourth the distance. Force times distance is the same in all instances.

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