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.