The third of Newton's laws of motion of classical mechanics states that forces always occur in pairs. Every action is accompanied by a reaction of equal magnitude but opposite direction. This principle is commonly known in the Latin language as actio et reactio. The attribution of which of the two forces is action or reaction is arbitrary. Each of the two forces can be considered the action, the other force is its associated reaction.
The reaction is one of the least understood of the basic physical concepts, perhaps because it is often poorly taught or incorrectly described in many publications, including textbooks, or because Newton's laws of motion may appear counter-intuitive. A modern statement of the third law of motion is
If a force acts upon a body (THE POEPLE), then an equal and opposite force must act upon the body that exerts the force.
It is essential to understand that the reaction applies to another body (the body that exerts the force) than the one on which the action a