Newton’s third law states: when object A exerts a force on object B, object B exerts an equal and opposite force on object A.
Properties of Newton’s third law pairs
Both forces:
- Are the same type (both gravitational, both contact, both magnetic, etc.)
- Have the same magnitude
- Act in opposite directions
- Act on DIFFERENT objects (this is crucial)
Examples
| Force A (on object B) | Force B (on object A) |
|---|---|
| Earth pulls apple downward (gravity) | Apple pulls Earth upward (gravity) |
| Foot pushes ground backward (contact) | Ground pushes foot forward (contact) |
| Rocket pushes exhaust gases backward | Exhaust gases push rocket forward |
The critical misconception
The weight of a book on a table and the normal contact force are NOT a Newton’s third law pair. They are both forces acting on the SAME object (the book), so they cannot be a third-law pair.
The correct pair for the weight of the book: the Earth pulls the book down; the book pulls the Earth up. Both forces are gravitational, same magnitude, opposite directions, on different objects.
Why Newton’s third law pairs do not cancel
They act on different objects. Only forces on the same object can be balanced or unbalanced. When you push a wall, the wall pushes you back with equal force, but the forces act on different objects.
Common errors and how to correct them
- Pairing weight and normal force (they act on the same object).
- Not specifying which object each force acts on.
- Saying the forces “cancel out” (they cannot, because they act on different objects).
How to apply this in an exam
Name both forces, state the type, say they are equal in magnitude, opposite in direction, and crucially, state which object each force acts on.
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