Newton's Laws for Kids - Third Law

A Simple Explanation of Principle of Action and Reaction

Jan 12, 2008

Isaac Newton discovered three laws of motion and the law of gravity that explain motions observed on Earth and in space. The third law deals with actions and reactions.

Newton's Third Law of Motion

You may be familiar with the words for Newton's third law: For every action there is an equal and opposite reaction. This statement is fairly well known, but there is a difference between memorizing the words and really understanding what they mean. The statement of this law is fairly easy, but the understanding is not. People often say the words without truly understanding their meaning.

Action-Reaction Pairs

You probably know that Earth orbits the Sun because a gravitational force acts between them, so try this question. Which is greater the gravitational force of the Sun on Earth or the gravitational force of Earth on the Sun? If you thought they were exactly the same, congratulations. If not, lets look at this question using Newton's third law.

The action and reaction in Newton's third law are forces. For every force there is an equal and opposite reaction force. The Sun's gravitational force acting on Earth is an action. The Newton's third law reaction is Earth's gravitational force acting on the Sun. Earth's gravitational force on the Sun and the Sun's gravitational force on Earth are an action-reaction pair.

Pair means two, so an action-reaction pair includes only two objects. There is never a third object in a Newton's third law action-reaction pair. If the Sun acts on Earth, the reaction is Earth acting on the Sun with an equal but opposite force.

For Newton's third law always remember action-reaction pairs, which are always two objects, never three.

This part often confuses people. Not all equal and opposite forces form a Newton's third law action-reaction pair.

If you weigh 120 pounds that means that Earth's gravity is pulling down on you with a force of 120 pounds. If you are standing on a solid floor, you do not accelerate downward because the floor is holding you up. The floor exerts an upward force of 120 pounds on you. These forces are equal and opposite, so you remain at rest on the floor.

Are these two equal and opposite forces an action-reaction pair? Let's look at them. The forces are Earth on you and floor on you. There are three objects involved, not two. So these forces are not a Newton's third law action-reaction pair. They are two different forces acting on you that happen to be equal and opposite.

The Newton's third law reaction to Earth pulling you down with a 120 pound force is that you pull Earth up with a 120 pound force. The forces, Earth on you and you on Earth, form an action-reaction pair.

The reaction to earth's gravity pulling you down, is you pull the Earth up with exactly the same force. So if the forces are the same, why do we fall down to Earth rather than having Earth fall up towards us? Earth is much more massive than we are. By Newton's second law a 120 pound force will affect us, but it will not significantly affect the much more massive Earth. So we fall down; Earth does not fall up.

For every action there is an equal and opposite reaction, but remember the action-reaction pairs.