Newton's Question: Who Moves?
One person grabs the rope and pulls. Nobody pushes. Three things could happen — and almost every audience picks the wrong one. Newton figured out why in 1687. Your students are about to discover it for themselves.
7-12 yrs
Easy
15
min
Stage 2, Stage 3
Mission Briefing.
Designed by Darin Carr (BSc, DipEd)
NESA Accredited Teacher Chemistry & Physics Specialist
Creator of the LAB™ Learning System
Professor Picklebottom
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Newton's Question: Who Moves?
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NESA Accredited Teacher
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High school chemistry & physics specialist 30+ years
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The Crazy Scientist in primary schools — 15 years
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International conference presenter on science education
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Creator of the LAB™ Learning System
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Curriculum aligned: NSW Science & Technology K–6 (2024)
A picture is worth a thousand words — check this out and see if you can spot the science hiding in plain sight.
Mission Equipment
2 rolling chairs (office or classroom chairs with wheels)
2 scooter boards
2 skateboards (works perfectly if available)Q
Let’s Investigate
1
Set Up
Place two rolling chairs (or boards or skateboards) facing each other on a smooth floor, approximately 2 metres apart.
One participant sits on each.
Stretch the rope between them so each person holds one end with both hands
3
Run the experiment
Nominate one participant as the puller.
The puller grabs their end of the rope and, on your signal, pulls steadily toward themselves.
The holder simply holds their end firmly — they do not pull, they do not push.
2
The prediction
Before anything happens, predict if one person pulls - who moves? (one: which one and /or both)
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The Crazy Scientist LAB Learning System™
Every experiment follows The Crazy Scientist Lab Learning System™ — a simple way to help kids think like real scientists.
We
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LINK to what they already know,
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ACTIVATE curiosity through hands-on discovery
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BUILD understanding that actually sticks.
In a tug of war — which team is pulling harder? The winning team, right?
Newton said no. Both teams pull the rope with exactly the same force. Every moment of the match.
If that's true — what actually wins a tug of war?
Today's experiment will show you. But first — predict:
Option A: The puller moves toward the holder. The holder stays still.
Option B: The holder moves toward the puller. The puller stays still.
Option C: Both move toward each other.
Write down your option and your reason.
You just watched both people slide toward each other — every time, no matter who pulled.
Think back to the exact moment the pull happened. Describe what you saw for each person — direction, speed, distance.
The holder never pulled anything. They just held on. Something moved them anyway.
What force acted on the holder — and where did it come from, if the holder didn't create it?
The lighter person travelled further, even though both people felt the same rope force. What is the variable that controlled how far each person moved?
You cannot push or pull something without it pushing or pulling back — equally, in the opposite direction.
That is Newton's Third Law. Forces always come in pairs. The puller and the holder are always a pair.
A rocket pushes exhaust gases backward at enormous speed. What pushes the rocket forward — and where does that force come from?
"Want the full teacher guide? The Crazy Scientist Lab includes classroom delivery tips, how to manage the WOW moment, differentiation for Stage 2 & 3, — ready to teach tomorrow."
Think Like a Scientist
Scientists don't just do ONE experiment; they change one part of the experiment (independent variable) and then see how it affects another part of the experiment
(dependent variable)
Change ONE variable and test again.
Does it make a difference if the two participants are very different sizes?
What happens if you run the experiment on carpet instead of a smooth floor?
🧪 Try it! Change ONE thing and test again. What did you discover?
Dr Puddledrip’s Science Tip
Want to go deeper? Tap a section below to explore. ▼
The Science Behind It
Why do both people move toward each other?
When the puller grabs the rope and pulls, they exert a force on the other person. But forces always come in pairs — you cannot pull something without it pulling back on you with exactly the same force, in the opposite direction.
The moment the puller pulls the rope, the rope pulls the puller back with equal force. Both forces exist at the same time, at the same strength.
Both people are on wheels with almost no friction. So both people move — toward each other — every single time. This is Newton's Third Law.
Why the holder moves even though they didn't do anything?
The holder appears passive — they just grip the rope. But the moment the puller applies a force through the rope, an equal and opposite force acts back on the puller, and the tension in the rope also acts on the holder.
The holder doesn't need to act. The act of pulling creates the reaction force automatically.
This is why Newton's Third Law is often misunderstood: both forces exist whether or not both people are trying. The action creates the reaction. You can't have one without the other.
Why the lighter person moves further?
The force on both people is identical — Newton's Third Law guarantees it. But the lighter person has less mass.
A smaller mass responds more to the same force:
acceleration = force ÷ mass.
Same force, less mass, greater acceleration. The lighter person travels further.
This is Newton's Second Law entering quietly: the Third Law sets the forces equal, but the Second Law shows that equal forces don't always produce equal motion.
Real-world connection
A rocket engine works entirely on Newton's Third Law. Burning gas is pushed downward out of the engine — the equal and opposite reaction pushes the rocket upward. There's nothing to push against in space. Pushing the gas away is enough.
Try next
• See Newton's First Law — what happens when a force stops acting → [The Runaway Planet]
• Explore how the same force produces very different results depending on mass → [The Coin Heist]
Extension: G&T Years 5 & 6
Vocabulary
Know a parent or teacher who'd love this? Send it on! 👇
READY TO TEACH THIS
TOMORROW?
Running the experiment is easy; however, teaching it well is another challenge.
Teachers often ask:
How do I adapt this for Stages 1,2 or 3?
What do I do with fast finishers?
What misconceptions will they have?
How do I structure this for a full class?
What syllabus outcomes does it cover?
What do I say when they ask WHY?
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