Physics 2.23 - Newton’s third law in equilibrium
Identify the two sides of a force interaction and explain why a third-law pair differs from balanced forces on one object. This equilibrium part of Physics 2.23 is required at both tiers.
Equilibrium and force pairs
This part is common to Foundation and Higher tiers. Begin by choosing the object whose motion is being considered. A force arrow on that object's force diagram must represent a force on that object.
Newton's third law
When object A exerts a force on object B, object B simultaneously exerts an equal-magnitude force in the opposite direction on object A.
A third-law pair is one interaction viewed from its two sides. The two forces:
- are the same type of interaction;
- have equal magnitudes and opposite directions;
- act at the same time;
- act on different objects.
Now compare that with equilibrium. A book resting on a horizontal table has an upward contact force from the table and a downward gravitational force (its weight). Both forces act on the book. If they are equal, their resultant on the book is zero, so the book remains at rest.
The upward contact force is not the third-law partner of the book's weight. Its partner is the downward contact force that the book exerts on the table. Those two contact forces act on different objects, so they cannot cancel in a force diagram for the book alone.
| Relationship | Upward force from table on book | Other force |
|---|---|---|
| Balancing pair for the book | Acts on the book | Weight acts downward on the book |
| Third-law interaction pair | Acts on the book | Book pushes downward on the table |
Continue this lesson.
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