A free-body diagram consists of a diagrammatic represent at ion of a single body or a substystem of bodies isolated from its surroundings but shown under the act ion of forces and moments due to external act ions.
Consider, for example, a book lying flat on a table. The book exerts its weight on the table and the table exerts its own weight as well as transmits the weight of the book on the ground. A free-body diagram for the book alone would consist of its weight W acting through the cent re of gravity and the react ion exerted on the book by the table top as shown in the figure.
A free-body diagram may be drawn for any single member of a system, any subsystem of the system or the entire system irrespective of whether the system is in equilibrium: at rest , in uniform motion or in a dynamic state of motion.