Interaction Pairs, Example: Consider a box resting on a table. (a) F1 is the force of the Earth on the box, what is the interaction partner of this force?

Answer

The interaction partner (or reaction force) is the force of the box pulling upward on the Earth. Newton's Third Law states that for every action, there is an equal and opposite reaction. These interaction pairs always involve the same two objects but in opposite directions. Since the gravitational force F1 is exerted by the Earth on the box (downward), its interaction partner must be exerted by the box on the Earth (upward). It is important to note that the normal force from the table is not the interaction partner of weight, even though they may be equal in magnitude, because they act on the same object (the box) rather than being a mutual interaction between two bodies.