IB Physics HLTopic 1 — Motion, Forces & EnergyPaper 1 & 2Equal & opposite force pairs~8 min read
Newton’s Third Law
The first two laws are about the forces acting on one object. The third law is different — it’s about what happens between two objects whenever they interact. Push on a wall and the wall pushes back on you, just as hard. Forces never come alone; they always arrive in matched pairs, equal in size and opposite in direction. It’s the reason you can walk, swim, and launch a rocket.
📘 What you need to know
Third law: if object A exerts a force on object B, then B exerts an equal and opposite force on A
Forces always come in pairs — often called third-law pairs
The two forces in a pair are the same type, same magnitude, opposite direction, and act on different objects
Because the pair acts on different objects, the two forces never cancel each other
Two equal-and-opposite forces on the same object are not a third-law pair (that’s the first law)
The third law explains walking, swimming, rockets — anything that moves by pushing on something else
What the law says
Here’s the statement:
Newton’s Third Law
If object A exerts a force on object B, then object B exerts a force on object A that is equal in magnitude but opposite in direction
The classic example is walking. Your foot pushes backward on the ground; the ground pushes forward on your foot with exactly the same size force. You feel the ground’s push, and that’s what drives you forward. The two forces are a matched pair — one on the ground, one on you.
The foot pushes back on the ground; the ground pushes forward on the foot with an equal and opposite force. The two arrows are the same length — and they act on different objects.
A neat trick for spotting a pair: describe the first force as “A pushes on B,” then just swap the names — “B pushes on A.” If you can do that swap and it describes a real force, you’ve found the partner. Foot pushes ground → ground pushes foot. Rocket pushes gas → gas pushes rocket. It works every time.
The four conditions of a pair
Not every “equal and opposite” pair of forces is a third-law pair. A genuine pair has to satisfy all four of these at once:
Miss any one of these and it isn’t a third-law pair. The last one — acting on different objects — is the one students forget most.
That last condition is the key one. Because the two forces act on different objects, they can never cancel each other out. The force on the ground and the force on your foot live on separate free-body diagrams entirely.
The trap: book on a table
Here’s where almost everyone slips. A book rests on a table. It has two forces on it: its weight pulling down, and the normal force from the table pushing up. They’re equal and opposite — so surely they’re a third-law pair? No. They both act on the same object (the book), so they fail the fourth condition. That balance is Newton’s first law, not the third.
The real third-law pairs are hiding elsewhere. There are actually two of them:
The real pairs: the table and book push on each other (contact), and the Earth and book pull on each other (gravity). Each pair is the same type of force, equal and opposite, on two different objects.
💡 Top tips
Same type of force. A contact push pairs with a contact push; a gravitational pull pairs with a gravitational pull. Never mix types.
Use the swap test. “A on B” must have a partner “B on A.” If you can’t phrase it that way, it’s not the pair.
Different objects, always. If your two forces act on the same object, they’re a first-law balance, not a third-law pair.
The weight of a person is the Earth pulling them — so its third-law partner is the person pulling the Earth, not the ground pushing up.
WE 1
A swimmer pushes backward against the water with a force of 250 N. Describe the third-law partner to this force, and explain what it does.
Step 1 — name the first force clearly
swimmer pushes water backward, 250 N
Step 2 — swap the objects for the partner
water pushes swimmer forward
Step 3 — apply the four conditions
same type (contact), same size = 250 N,
opposite direction, different object (water → swimmer)
water pushes swimmer forward, 250 NIt’s this forward push from the water on the swimmer that drives them through the pool.
WE 2
A physics textbook rests on a bench. A student claims the book’s weight and the upward normal force from the bench are a Newton’s third-law pair. Is the student correct? Explain.
Step 1 — check the four conditions
weight (down) and normal (up) are equal & opposite…
Step 2 — but which objects do they act on?
BOTH act on the book → fails “different objects”
also different types (gravity vs contact)
no — not a third-law pairThey balance because of Newton’s FIRST law. The true partners are: book pushes bench down, and book pulls Earth up.
Quick recap: forces come in pairs — if A pushes B, then B pushes A, equally and oppositely. A real third-law pair is the same type of force, equal in size, opposite in direction, and acts on two different objects. Two balanced forces on the same object are the first law, not the third.
⚠ Common mistakes
Calling a book’s weight and normal force a third-law pair — they act on the same object, so they can’t be
Pairing forces of different types (e.g. a gravitational force with a contact force)
Thinking the pair cancels out — they act on different objects, so they never appear on the same free-body diagram
Forgetting that the partner to a person’s weight is the person pulling the Earth, not the floor pushing up
Assuming the bigger object “wins” — the two forces are always exactly equal, whatever the masses
That completes Newton’s three laws — you can now describe the forces on a single object and the pairs between two. Next we start naming the forces themselves: the ones that only act when objects touch. Up next is Contact Forces — friction, tension, drag, and the normal reaction.
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