Newton’s Third Law

A book sits on the table. A tennis racquet and tennis ball come into contact. A fly finds its way in front of the windshield of a speeding automobile. Each of these is an interaction between two objects. Whenever two objects interact, forces are involved. Newton’s third law tells us how these forces relate to one another.

Newton’s Third Law:

When two objects interact, each object exerts a force on the other object. 

These forces will:

• Be the same type of force
• Be of Equal Magnitude
• Act in Opposite Directions
• Act at the same time

If you push against a wall, the wall pushes back.  If your force is fifty Newtons into the wall, the wall will be pushing you with a force of fifty Newtons. 

Some refer to Newton’s third law as being about action and reaction.  This implies that first you push on the wall (action) then the wall pushes back (reaction).

The risk with this description is that it might imply that one happens before the other.  Yet, the very instant that you start to push against the wall is the exact instant that the wall starts to push against you.  As soon as you stop pushing into the wall, the wall stops pushing against you.

Take a rubber band and loop one end around a fixed object, and the other end around your index finger.  Pull lightly on the rubber band and you’ll notice that the rubber band pulls back lightly.  Increase the force you are applying to the rubber band, and the rubber band simultaneously responds with a greater force.  In fact, the force of your finger on the rubber band is equal in magnitude and opposite in the direction to the force of the rubber band on your finger.

You can see, therefore, that Third Law Forces come in pairs. Look at a situation to see what you can discover about these force pairs.

A book sits on the table. Describe the forces involved.

You might say that gravitational force pulls the book down and the normal force from the table pushes the book up.  This would be sufficient for a free body diagram, however these are not a pair of Newton’s Third Law forces.

In order for forces to be a pair, they must be the same type of force.  Also, they must act on different objects.

So one force pair in this situation involves gravitational forces.  The gravitational force of the earth on the book pulling the book down is paired with the gravitational force of the book on the earth pulling the earth up.

The other force pair here is a normal force pair.  The normal force of the table on the book pushes the book up while the normal force of the book on the table pushes the table down.

When you are drawing free body diagrams, you are interested in the individual forces acting on a single object.  When you are identifying force pairs, you are interested in pairs of forces acting on two different objects.

In a free body diagram, forces may be balanced or unbalanced.  Newton’s third law force pairs are always balanced.

Look at a boy pushing a box along the floor at a constant velocity. How many Newton’s Third Law force pairs can you identify?

• The boy pushes on the box and the box pushes on the boy.
• The box pushes down on the floor with a normal force.  The floor pushes up on the box with a normal force.
• The gravitational force of the earth acts downward on the boy and the gravitational force of the boy acts upward on the earth.
• The gravitational force of the earth acts downward on the box and the gravitational force of the box acts upward on the earth.
• A static friction force between the boy’s shoes and the floor acts to the left on the floor and to the right on the boy.
• A kinetic friction force between the sliding box and the floor acts to the left on the box and acts to the right on the floor.

Each pair is two forces of the same type and of the same magnitude acting in opposite directions on two different objects.

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Newton's Third Law Practice

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