Newton's third-law pairs
Forces arise from interactions between two bodies. Newton's third law links the force of A on B to the force of B on A.
Name both bodies in the interaction
Definition
What is meant by Newton's third law?
Model answer: When body A exerts a force on body B, body B exerts an equal force in the opposite direction on body A.
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- The two forces have equal magnitude.
- They act in opposite directions along the same line.
- They are the same type of interaction.
- They act on different bodies.
- They act at the same time; one does not happen after the other.
Separate third-law pairs from balanced forces
Worked example
A person standing still
Earth pulls a person downward with weight 600 N. The third-law partner is the person pulling Earth upward gravitationally with 600 N.
The floor's 600 N normal reaction balances the person's weight, but these two forces are not a third-law pair: both act on the person and arise from different interactions.
Common mistake
Internal pairs cancel only for a whole system
When two touching or connected bodies are treated as one system, the interaction forces between them are internal and cancel in the system equation. When either body is isolated, its interaction force must appear on that body's free-body diagram.
- State the selected body or system.
- Keep every external force on that selection.
- Remove only interaction pairs that are internal to the selected whole system.
A person of weight 520 N stands at rest on a floor. State (a) the magnitude and direction of the force of the floor on the person, and (b) the third-law partner of the person's weight.
Show worked answer
The person is in equilibrium, so the floor exerts a 520 N normal reaction upward on the person.
Weight is Earth's gravitational force on the person. Its third-law partner is the 520 N gravitational force of the person on Earth, directed toward the person.
(a) upward on the person.
(b) The person exerts a gravitational force of on Earth, directed toward the person.