Section 2 Chapter 16 Matter in Motion This section defines force and gives examples of different kinds of forces. We will learn to determine the net force on an object and to compare balanced and unbalanced forces. In science , a force is simply a push or pull. All forces have both size and direction. Forces are everywhere, anytime you see an object in motion, you can be sure that its motion was created by force. Scientists express force using a unit called the newton (N). The more newtons, the greater the force. All forces are exerted by one object on another object. For any push to occur, something has to receive the push. The same is true for any pull. It is not always easy to tell what is exerting a force or what is receiving a force. You cannot see what exerts the force that pulls magnets to refrigerators. The air you breathe is an unseen receiver of a force called gravity. Often more than one force is exerted on an object at the same time. The net force is the force that results from combining all the forces exerted on an object. To calculate net force when the forces are in the same direction, you just the forces together. When forces are in different directions, you subtract the smaller force from the larger force. When a net force on an object is not zero, the forces on the object are unbalanced. Unbalanced forces produce a change in motion (acceleration). Unbalanced forces are necessary to cause a nonmoving object to start moving. They also are necessary to change the motion of moving objects. An object can continue to move even when the unbalanced forces are removed. When the forces applied to an object produce a net force of zero, the forces are balanced. Balanced forces do not cause a nonmoving object to start moving, nor will it cause a change in the motion of a moving object. Many objects around you have only balanced forces acting on them.