--- title: "Push the wall, and it pushes you?" subtitle: "Year 10 Physics · Newton's laws · Lesson 5" --- # Push the wall, and it pushes you? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Stand on a skateboard and push a wall. You roll away from it. The wall didn't move, so what pushed you? :::teacher-note **In one sentence:** 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). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **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 My prediction, and why: {{lines-3}} ::: :::teacher-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. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: pairs of forces :::keep-together ### 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 ::: :::teacher-note **Answer:** When A exerts a force on B, B exerts an equal and opposite force on A Forces come in pairs: same size, same type, opposite directions, on different objects. ::: :::keep-together ### Partners **Match each force to its partner in the pair.** A. You pull the Earth up B. The water pushes the swimmer forward C. The gas pushes the rocket up D. The ground pushes forward on your foot {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Your foot pushes back on the ground` | | | `A rocket pushes gas down` | | | `The Earth pulls you down` | | | `A swimmer pushes water back` | | ::: :::teacher-note **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 Swap the two objects and reverse the direction: that's the partner. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** No: both act on the book. The partner of the book's weight is the book pulling the Earth up 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. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** Neither: the forces on each are the same size Third law: the forces are equal. The car, with less mass, accelerates more (F = ma), which is why it fares worse. ::: :::keep-together ### 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 ::: :::teacher-note **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 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. ::: :::typical ## Elaborate: the book :::keep-together ### 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 ::: :::teacher-note **Answer:** The book pushing the table down Swap the two objects and reverse the direction. ::: ::: :::wider ## Elaborate: walking :::keep-together **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 ::: :::teacher-note **Answer:** You can't push back on the ice hard enough, so it can't push you forward hard enough 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. ::: ::: ## 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). ## Exit ticket :::keep-together **A swimmer pushes the water backwards. Name the other force in the pair, and say which way it acts and on what.** {{lines-3}} ::: :::teacher-note **Answer:** The water pushes the swimmer forwards, on the swimmer, with a force of the same size. :::