ACTION AND REACTION PRINCIPLE 

  

What Are Action Forces?

To begin, forces always act in pairs and always act in opposite directions. When you push on an object, the object pushes back with an equal force. Think of a pile of books on a table. The weight of the books exerts a downward force on the table. This is the action force. The table exerts an equal upward force on the books. This is the reaction force. Note that the two forces act on different objects. The action force acts on the table, and the reaction force acts on the books.

Force Pairs and Newton's Third Law

Newton's Third Law states that forces always act in pairs. Consider an example of a boy playing with a dog's toy and what it illustrates. There is a force from the boy on the dog's toy, and there is a force from the dog's toy on the boy. These two forces create an interaction pair. Forces always come in pairs similar to this example. Consider the boy (A) as one system and the toy (B) as another. What forces act on each of the two systems? Picture the boy pulling on a toy and the toy being pulled from the boy. You can see that each system exerts a force on the other. The two forces - F(A on B) and F(B on A) - are the forces of interaction between the two. Notice the symmetry in: A on B and B on A.

The forces F(A on B)and F(B on A) are an interaction pair, which is a set of two forces that are in opposite directions, have equal magnitudes and act on different objects. Sometimes, an interaction pair is called an action-reaction pair. This might suggest that one causes the other; however, this is not true. For example, the force of the boy pulling on the toy doesn't cause the toy to pull on the boy. The two forces either exist together or not at all.

There can never be a single force acting alone. Forces only come in action-reaction pairs. Think carefully about propelling a skateboard with your foot. Your foot presses backward against the ground. The force acts on the ground. However, you move, so a force must act on you, too. Why do you move? What force acts on you? You move because the action force of your foot against the ground creates a reaction force of the ground against your foot. You 'feel' the ground because you sense the reaction force pressing on your foot. The reaction force is what makes you move because it acts on you.