Forces come in pairs. When object A pushes or pulls object B, B pushes or pulls A back with a force of the same size in the opposite direction. The two forces act on different objects, so they never cancel each other out.
Stand on a skateboard and push a wall. You roll away from it. The wall didn't move, so what pushed you?
You know a net force changes motion. This lesson asks where forces come from: they always come in pairs.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: the skater and the wall
ExploreTry it on the board and compare what happens with your prediction.
Explore: the wall pushes back
The board shows only the forces on the skater. Her push on the wall acts on the wall, so it isn't drawn here.
Try it yourselfYour edits are saved in this browser.
Press Play while the wall pushes, then set its push to 0 (she lets go). Let her glide for a few seconds.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: pairs of forces
In your book: draw the skater and the wall as two separate pictures. On each, draw only the force acting on it. Label the pair "equal size, opposite directions".
Match each force to its partner in the pair.
Your foot pushes back on the ground
A rocket pushes gas down
The Earth pulls you down
A swimmer pushes water back
Elaborate TypicalUse the same idea in a new context.
Elaborate: the book
Try it yourselfYour edits are saved in this browser.
Set the table's push so the forces on the book balance.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: walking
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Newton's third law: when object A pushes or pulls object B, B pushes or pulls A with a force of the same size in the opposite direction.
The two forces in a pair act on different objects, so they never cancel each other.
Balanced forces act on the same object (weight and support on a book); they are not a third-law pair.
You move by pushing on something else: it pushes you (walking, swimming, rockets).
For teachers
Students state Newton's third law, identify the two forces in an action–reaction pair (who on whom), and distinguish third-law pairs from balanced forces on one object (AC9S10U05, node 5). About 24 minutes.
I can name both forces in an action–reaction pair.
I can tell a third-law pair from two balanced forces on one object.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider. It follows F = ma and comes before free body diagrams (node 6).
Not a hinge node, but its classic error matters: calling weight and the support force a third-law pair. The book question and the Typical step target it.
Board notes: the force board draws the forces on one object only, which is the point: a third-law partner always acts on the other object, so it is never on the same diagram. The skater's push from the wall is a steady force while she touches it; real pushes rise and fall.
Running the lesson
Engage: tally the predictions. 'Her own push' is the idea to work on.
Explore: ask which force on the board acts on her, and where her own push went.
Explain: the partners match and the book question are the success criteria.
In your own words: two separate pictures, one force on each.
The hard part (on purpose)
Naming who pushes on whom. When a student calls two forces a pair, ask them to say each force as 'A on B' out loud before deciding.
When students get stuck
A student says weight and the table's push are a third-law pair. Ask: What is each force acting on? Fix: Both on the book: they balance. Pairs act on two different objects.
A student thinks the heavier object exerts the bigger force in a collision. Ask: What does the third law say about the sizes? Fix: Equal. The lighter object accelerates more because of its smaller mass.
A student says the pair cancel, so nothing can ever move. Ask: Are both forces on the same object? Fix: No. Only forces on the same object add up to a net force; each object feels just one of the pair.
Stretch and support
Stretch: A horse pulls a cart, and the cart pulls back on the horse equally. How does anything move? (The horse pushes back on the ground and the ground pushes it forward harder than the cart pulls back; look at the forces on the horse only.)
Support: Give the frame: '… pushes/pulls … (force 1). … pushes/pulls … back (force 2). Same size, opposite directions, different objects.'
Where it leads
Next: What forces act on it? (free body diagrams, node 6). Integrating all three laws (nodes 7 and 8) is a class task.
In class: two students on wheeled chairs pushing off each other; balloon rockets on a string.
What each step is for
Engage: the skater and the wall: Students who think the skater moves 'because she pushed' haven't placed the force on her; the third law puts the wall's push on the skater.
Explore: the wall pushes back: Seeing the only force on the skater is the wall's push makes the 'other half' of the pair the one that changes her motion.
Explain: pairs of forces: Naming both forces in a pair (who on whom), then telling a third-law pair from two balanced forces on one object, is the hinge of the law.
Elaborate: the book: Balancing the book, then naming the partner of each force, separates 'balanced' from 'pair' on a familiar example.
Elaborate: walking: Walking and rockets show the third law is how anything starts moving: by pushing on something else.
Now you can
State Newton's third law.
Name both forces in an action–reaction pair.
Tell a third-law pair from two balanced forces on one object.