What is the key principle stated by Newton's Third Law?

Newton's Third Law states that for every action, there is an equal and opposite reaction. This principle highlights the relationship between two interacting objects: when one object exerts a force on another, the second object exerts a force of equal magnitude but in the opposite direction on the first object. This concept is fundamental in understanding how forces work in pairs and is evident in various physical phenomena, such as the way a rocket propels itself by expelling gas downwards, which results in the rocket being pushed upwards.

In the context of the other options, while forces indeed act upon each other, the remark about forces existing in isolation does not align with the notion of action and reaction. Claiming all forces are equal in magnitude neglects the directional aspect and the specific interaction context described by Newton’s law. As for energy, while it plays a crucial role in physical processes, the assertion that energy cannot be created or destroyed refers to the conservation of energy, which is a different principle from Newton’s laws of motion.