In studying the effect of forces on a body it is necessary to isolate the body and determine all forces acting upon it. This method of using a free-body diagram is essential in understanding basic and complex force problems.

EO 1.2STATE the purpose of a free-body diagram.

EO 1.3Given all necessary information, CONSTRUCT a free-body diagram.

In solving a problem involving forces it is essential that Newton's laws are carefully fixed in mind and that these principles are applied literally and exactly. In applying these principles it is essential that the body be isolated from all other bodies so that a complete and accurate account of all forces which act on this body may be considered. The diagram of such an isolated body with the representation of all external forces acting on it is called aFree-Body Diagram. It has long been established that the free-body-diagram method is the key to the understanding of engineering problems. This is because the isolation of a body is the tool that clearly separates cause and effect and focuses our attention to the literal application of a principle.

Example:

Consider the book resting on the table in Figure 1. Although the book is stationary, two forces are acting on the book to keep it stationary. One is the weight (W) of the book exerting a force down on the table. The other is the force exerted up by the table to hold the book in place. This force is known as the normal force (N) and is equal to the weight of the book. A normal force is defined as any perpendicular force with which any two surfaces are pressed against each other. The free-body diagram for this situation, illustrated on the right side in Figure 1, isolates the book and presents the forces acting on the object.

Figure 1 Book on a Table