Free body

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Free body is the generic term used by physicists and engineers to describe some thing—be it a bowling ball, a spacecraft, pendulum, a television, or anything else—which can be considered as moving as a single unit. The object doesn't have to be "free" in the usual sense of the word—it could be completely prevented from going anywhere, or it could be trapped in an orbit. The crucial element is that the physicist can think of it as a single unit to the extent that it either does or does not move, as the case may be.

A free body possesses two fundamental properties that make the concept useful in analyzing static and dynamic equilibrium:

• The internal forces of the free body are, by definition, assumed to completely off-set one another so that the body as a whole is in static equilibrium.

• As a consequence, the body can be treated as a single entity with its total mass located entirely at the center of mass and subject to Newton's second law