Free body is the generic term used by physicists and engineers to describe an object—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. To study the stresses acting in a material, small elements or sections of beams are treated as free bodies. The crucial concept 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. Often the term is associated with a free body diagram.