Newton's Law of Motion (Newton's Third Law)

Action-Reaction

If we press a wood block with our fingers, we will feel hurt. Why is that? The pain is actually caused by the force from the wood to our fingers. The amount of force exerted to our finger is the same as the amount of force exerted by our fingers to the wood. The harder we push the wood the more hunt our fingers will be. This phenomenon indicates the existence if action-reaction. Because the finger exerts a force o the wood, the wood exerts a force on the figer with the same magnitude but in opposite direction. The force by the figer is called action, and the force by wood is reaction. This phenomenon is concluded in the Newton’s third law as follows.

If the first object exerts a force on the second object, then the object will exerts the same amount of force to the first object in the opposite direction.

F_action  = - F_reaction

The negative sign indicates that F_action and F_reaction are in opposite direction.

The forces of action and reaction are always the same, in opposing direction, at the same incident point, and work on two different objects.

The coupling force of action - reaction