Newton’s third law, also known as the principle of action and reaction, expresses the always mutual nature of forces. No body can exert a force on another without in turn experiencing an equal and opposite one. Forces, in other words, never exist on their own: they exist only as interactions between pairs of bodies.
Principle — Newton's third law
If a body A exerts on a body B a force , then body B exerts on A an equal and opposite force:
The two forces — which we call twin forces — have the same magnitude, the same line of action, and opposite sense. In the language of interaction diagrams, every squiggle connecting two bodies represents exactly one of these pairs: a single interaction, two twin forces. The crucial point, and the source of almost all mistakes, is that the two forces act on different bodies: one on A, the other on B. For this reason they are never added together and never appear in the same Newton equation.
The third law: a squiggle represents two equal and opposite twin forces. Each acts on a different body: they never appear in the same Newton equation.
Common mistakes
- Twin forces act on different bodies: they are never added together, and in particular they do not cancel out (acting on distinct bodies, they cannot balance each other).
- The two twins are always of the same type: if one is gravitational in nature, so is the other; if one is a tension, so is the other.
- The floor’s constraint reaction is not the twin of the weight of a body resting on it. The true twin of the weight is the gravitational force that the body exerts on the Earth. Weight and normal reaction both act on the same body, so they cannot be an action–reaction pair: they are, if anything, two forces that balance by the second law, not by the third.
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Topics: Dinamica Concepts: Terza legge di Newton
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