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# Push the wall, and it pushes you?

Year 10 Physics · Newton's laws

<!-- Speaker note: 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 (Engage and Explore 7 min · Explain 10 min · Evaluate 3 min · Typical adds 3 min, Wider about 2 min). -->

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## Today

**We are learning to** 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.

- I can state Newton's third law.
- I can name both forces in an action–reaction pair.
- I can tell a third-law pair from two balanced forces on one object.

<!-- Speaker note: 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. Teacher: In your own words: two separate pictures, one force on each. -->

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## Why it matters

> Stand on a skateboard and push a wall. You roll away from it. The wall didn't move, so what pushed you?

<!-- Speaker note: You know a net force changes motion. This lesson asks where forces come from: they always come in pairs. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**A skater stands still and pushes on a wall. She rolls backwards. What force made her move?**

- Her muscles pushing her backwards
- Her own push on the wall
- The wall pushing back on her
- Nothing: she moved without a force

<!-- Speaker note: What happens: The wall pushing back on her. Her push acts on the wall (which is too heavy to move). By Newton's third law the wall pushes back on her with a force of the same size, and that force, on her, is what moves her. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. 'Her own push' is the idea to work on. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-10%2Fthird-law%2Fstep-2&code=widget%3A+forces%0Aobject%3A+Skater%0Amass%3A+50%0Aforce+P%3A+right%2C+100%2C+The+wall+pushing+the+skater%2C+adjust+0-200+by+50%0Amotion%3A+on%0Aspeed%3A+0%0Atrack%3A+20%0Atime%3A+6%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-10/third-law/?full=step-2#step-2)

<!-- Speaker note: The board shows only the forces on the **skater**. Her push on the wall acts on the wall, so it isn't drawn here. -->

<!-- Speaker note: Teacher: ask which force on the board acts on her, and where her own push went. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## What happened?

* The wall pushing back on her. Her push acts on the wall (which is too heavy to move). By Newton's third law the wall pushes back on her with a force of the same size, and that force, on her, is what moves her.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The law

**What does Newton's third law say?**

- Bigger objects push harder
- When A exerts a force on B, B exerts an equal and opposite force on A
- Every force is balanced by another force on the same object

* Answer: When A exerts a force on B, B exerts an equal and opposite force on A

<!-- Speaker note: Forces come in pairs: same size, same type, opposite directions, on different objects. -->

<!-- Speaker note: Teacher: the partners match and the book question are the success criteria. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Partners

**Match each force to its partner in the pair.**

| | |
|---|---|
| `Your foot pushes back on the ground` | A. You pull the Earth up |
| `A rocket pushes gas down` | B. The water pushes the swimmer forward |
| `The Earth pulls you down` | C. The gas pushes the rocket up |
| `A swimmer pushes water back` | D. The ground pushes forward on your foot |

* Answer: Your foot pushes back on the ground → D. The ground pushes forward on your foot; A rocket pushes gas down → C. The gas pushes the rocket up; The Earth pulls you down → A. You pull the Earth up; A swimmer pushes water back → B. The water pushes the swimmer forward

<!-- Speaker note: Swap the two objects and reverse the direction: that's the partner. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## The book on the table

**A pile of books rests on a table. Its weight is 49 N down; the table pushes it up with 49 N. Are these a third-law pair?**

- No: both act on the book. The partner of the book's weight is the book pulling the Earth up
- Yes: they are equal and opposite
- Yes: they cancel out

* Answer: No: both act on the book. The partner of the book's weight is the book pulling the Earth up

<!-- Speaker note: Balanced forces act on one object. A third-law pair always acts on two objects: Earth pulls book, book pulls Earth; table pushes book, book pushes table. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Truck and car

**A truck and a small car collide. Which feels the bigger force?**

- Neither: the forces on each are the same size
- The truck, because it is heavier
- The car, because it gets damaged more

* Answer: Neither: the forces on each are the same size

<!-- Speaker note: Third law: the forces are equal. The car, with less mass, accelerates more (F = ma), which is why it fares worse. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Horse and cart

**A horse pulls a cart forward with 500 N. By the third law, the cart pulls back on the horse with 500 N. So how can the cart start moving?**

- It can't: equal and opposite forces cancel, so nothing moves
- The horse pulls a little harder than the cart pulls back
- The two forces act on different objects, so they don't cancel: the cart speeds up if the horse's pull is bigger than the friction on the cart

* Answer: The two forces act on different objects, so they don't cancel: the cart speeds up if the horse's pull is bigger than the friction on the cart

<!-- Speaker note: To see what the cart does, add only the forces on the cart: the horse's pull forward and friction back. The cart's pull acts on the horse, not the cart. -->

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## Elaborate: the book

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-10%2Fthird-law%2Fstep-4&code=widget%3A+forces%0Aobject%3A+Books%0Amass%3A+5%0Ag%3A+9.8%0Aforce+W%3A+down%2C+weight%2C+Earth+pulling+the+book+%28weight%29%0Aforce+N%3A+up%2C+0%2C+Table+pushing+the+book+up%2C+adjust+0-80%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-10/third-law/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Balancing the book, then naming the partner of each force, separates 'balanced' from 'pair' on a familiar example. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Find the partner

**What is the partner of 'the table pushing the book up'?**

- The book pushing the table down
- The table pulling the Earth
- The Earth pulling the book down

* Answer: The book pushing the table down

<!-- Speaker note: Swap the two objects and reverse the direction. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: walking

**Why is it hard to walk on very slippery ice?**

- Ice is too cold for your muscles
- You can't push back on the ice hard enough, so it can't push you forward hard enough
- You push on the ice hard, but it pushes back on you less hard

* Answer: You can't push back on the ice hard enough, so it can't push you forward hard enough

<!-- Speaker note: To walk forward you push back on the ground and it pushes you forward. With little friction, your backward push slips, and so does the forward push you get. What this step is for: Walking and rockets show the third law is how anything starts moving: by pushing on something else. -->

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## 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).

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**A swimmer pushes the water backwards. Name the other force in the pair, and say which way it acts and on what.**

* The water pushes the swimmer forwards, on the swimmer, with a force of the same size.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this 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. -->