What Is a Force?

Forces in the Same Direction

Suppose the music teacher asks you and a friend to move a piano. You pull on one end and your friend pushes on the other end. The forces you and your friend exert on the piano act in the same direction. The two forces are added to determine the net force because the forces act in the same direction. In this case, the net force is 45 N. This net force is large enough to move the piano—if it is on wheels, that is!

Determining Net Force

Usually, more than one force is acting on an object. The net force is the combination all of the forces acting on an object. So, how do you determine the net force? The answer depends on the directions of the forces.

Balanced Forces

When the forces on an object produce a net force of 0 N, the forces are balanced. Balanced forces will not cause a change in the motion of a moving object. And balanced forces do not cause a non moving object to start moving. Many objects around you have only balanced forces acting on them. For example, a light hanging from the ceiling does not move because the force of gravity pulling down on the light is balanced by the force of the cord pulling upward. A bird's nest in a tree and a hat resting on your head are also examples of objects that have only balanced forces acting on them.

Unbalanced Forces

When the net force on an object is not 0 N, the forces on the object are unbalanced. Unbalanced forces produce a change in motion, such as a change in speed or a change in direction. Unbalanced forces are necessary to cause a non moving object to start moving. Unbalanced forces are also necessary to change the motion of moving objects. For example it is a soccer game; the soccer ball is already moving when it is passed from one player to another. When the ball reaches another player, that player exerts an unbalanced force—a kick—on the ball. After the kick, the ball moves in a new direction and has a new speed. An object can continue to move when the unbalanced forces are removed. For example, when it is kicked, a soccer ball receives an unbalanced force. The ball continues to roll on the ground long after the force of the kick has ended.

Forces in Different Directions

When two forces act in opposite directions, you subtract the smaller force from the larger force to determine the net force. The net force will be in the same direction as the larger force.

Section Summary

A force is a push or a pull. Forces have size and direction and are expressed in newtons. Force is always exerted by one object on another object. Net force is determined by combining forces. Forces in the same direction are added. Forces in opposite directions are subtracted. Balanced forces produce no change in motion. Unbalanced forces produce a change in motion.

Forces Acting on Objects

All forces act on objects. For any push to occur, something has to receive the push. You can't push nothing! The same is true for any pull. When doing schoolwork, you use your fingers to pull open books or to push the buttons on a computer keyboard. In these examples, your fingers are exerting forces on the books and the keys. So, the forces act on the books and keys. However, just because a force acts on an object doesn't mean that motion will occur. For example, you are probably sitting on a chair. But the force you are exerting on the chair does not cause the chair to move. The chair doesn't move because the floor is also exerting a force on the chair.

Balanced and Unbalanced Forces

If you know the net force on an object, you can determine the effect of the net force on the object's motion. Why? The net force tells you whether the forces on the object are balanced or unbalanced.

Introduction

In science, a force is simply a push or a pull. All forces have both size and direction. A force can change the acceleration of an object. This acceleration can be a change in the speed or direction of the object. In fact, any time you see a change in an object's motion, you can be sure that the change in motion was created by a force. Scientists express force using a unit called the newton (N).

Unseen Sources and Receivers of Forces

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. And you cannot see that the air around you is held near Earth's surface by a force called gravity.