Knowledge of the forces required to maintain an object in equilibrium is essential in understanding the nature of bodies at rest and in motion.

EO 1.4STATE the conditions necessary for a body to be in force equilibrium.

EO 1.5DEFINE the following:

a. Net force

b. Equilibrant

Net Force

When forces act on an object, the result may be a change in the object's state of motion. If certain conditions are satisfied, however, the forces may combine to maintain a state of equilibrium or balance.

To determine if a body is in equilibrium, the overall effect of all the forces acting on it must be assessed. All the forces that act on an object result in essentially one force that influences the object's motion. The force which results from all the forces acting on a body is defined as the net force. It is important to remember that forces are vector quantities. When analyzing various forces you must account for both the magnitude (displacement) of the force as well as the direction in which the force is applied. As described in the previous chapter, this is best done using a free-body diagram.

To understand this more clearly, consider the book resting on the table in section A of Figure 5.

Figure 5 Net Force

The book remains stationary resting on the table because the table exerts a normal force upward equal to the weight of the book. Therefore, the net force on the book is zero. If a force is applied to the book (section B of Figure 5), and the effect of friction is neglected, the net force will be equal to the applied force, and the book will move in the direction of the applied force. The free-body diagram in section C of Figure 5 shows that the weight (W) of the book is canceled by the normal force (N) of the table since they are equal in magnitude but opposite in direction. The resultant (net) force is therefore equal to the applied force (F_APP ).