The first two laws are about the forces on a single object. The third law is about what happens when two objects interact: every force comes with an equal and opposite partner acting on the other object. Get the “pair” criteria right and you’ll never fall for the classic book-on-a-table trap.
📘 What you need to know
The law: if object A exerts a force on object B, then B exerts a force on A that is equal in size but opposite in direction.
These two forces are a third-law (action–reaction) pair.
A genuine pair is: the same type of force, the same magnitude, in opposite directions, acting on different objects.
Two equal-and-opposite forces on the same object are not a third-law pair — that’s Newton’s first law.
Handy framework: “A pushes/pulls B, and B pushes/pulls A.”
What the law says
Newton’s third law
If A exerts a force on B, then B exerts an equal and opposite force on A.
Forces always come in pairs because they arise from an interaction between two objects. You can never have a single, lonely force — push on something and it pushes back just as hard the other way.
Equal in size, opposite in direction, and acting on two different objects.
When is it a third-law pair?
Not every pair of equal, opposite arrows is an action–reaction pair. Run through all four checks — if any one fails, it isn’t a third-law pair:
✅ The four-point check
Are they the same type of force? (both contact, or both gravitational, etc.)
Are they the same magnitude?
Do they point in opposite directions?
Do they act on different objects?
An everyday example: walking
You move forward because your foot pushes backwards on the ground, and the ground pushes your foot forwards by the same amount. Both are normal contact forces (the same type), equal in size, opposite in direction, and acting on different objects — the foot and the ground.
The foot pushes the ground back; the ground pushes the foot forward. Same type, equal, opposite, different objects ✓
Worked example: the book on a table
WE 1
Which law does the diagram show?
A book rests on a table. A free-body diagram of the book shows its weight acting down and the normal force from the table acting up. Student A says this is an example of Newton’s third law; Student B says it’s Newton’s first law. Who is right?
Check the two forces on the book: weight + normaldifferent types (gravitational vs contact), and both act on the SAME objectSo they fail the four-point check → not a third-law pairthey balance, keeping the book at rest → that’s the 1st lawStudent B is correct
So where are the real third-law pairs? You have to look at the interactions between two objects:
The genuine pairs: book ↔ table (both contact forces) and book ↔ Earth (both gravitational). Each pair is the same type, equal, opposite, and on different objects.
Quick reference: third-law pair = same type + same size + opposite direction + different objects. Two forces on one object are never a pair — that’s the first law.
💡 Top tips
Name the objects. Write “X pushes/pulls Y” and “Y pushes/pulls X” — if you can’t, it isn’t a clean pair.
Same type matters. The partner of a weight (gravitational) force is another gravitational force, not a contact force.
Different objects, always. The two forces in a pair never act on the same body.
Simplify first. Start by calling each force just “a push” or “a pull”, then add the proper name once you’ve matched the pair.
⚠ Common mistakes
Calling weight and normal force a pair. They act on the same object and are different types — that’s the first law, not the third.
Matching a gravitational force with a contact force. A pair must be the same type.
Assuming equal-and-opposite always means a pair. It only does if all four criteria hold.
Thinking the two forces cancel. They act on different objects, so they can’t cancel each other out.
The normal force from the book on the table happens to equal the book’s weight — which is exactly why people wrongly pair them. They’re equal for a first-law reason (the book is in equilibrium), not because they’re an action–reaction pair. Up next: Contact Forces — a closer look at friction, drag, tension and the normal force.
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