--- title: "Levers" subtitle: "Mechanisms · Levers and pulleys · Lesson 3" --- # Levers **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A 600 N rock, a 2 m bar, and a block to rest it on. You can push 200 N at most. Where does the block go? :::teacher-note **In one sentence:** Students slide a pivot along a bar until a 200 N push lifts a 600 N rock, learn that turning effects (force × distance from the pivot) balance and that the hand moves farther by the same factor, meet class 2 and class 3 levers in a wheelbarrow and their own arm, and design a lever that meets two competing criteria. **Students should be able to say:** I can work out the push a lever needs from force × distance on each side of the pivot. I can explain that a lever makes my push bigger by the same factor it makes my hand move farther. I can say whether a lever is class 1, 2 or 3 from where the pivot, the load and the push are. I can design a lever that meets more than one criterion. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **The pivot starts in the middle of the bar. You move it closer to the rock. The push you need…** ☐ Stays the same: the rock weighs the same ☐ Gets bigger ☐ Is smallest with the pivot in the middle ☐ Gets smaller My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Smaller. With the pivot near the rock, your push is far from the pivot and the rock is close to it, so each newton of push turns the bar more. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: turning effects balance :::keep-together ### Balance 600 N with the pivot at 0.5 m, then 0.4 m 1. $600 \times 0.5 = 300$ — The rock's turning effect: its force times its distance from the pivot. 2. $2 - 0.5 = 1.5 \text{ m}$ — Your push is at the other end: 1.5 m from the pivot. 3. $300 \div 1.5 = 200 \text{ N}$ — Your push must turn as much: push × 1.5 = 300. 4. $\text{advantage} = 600 \div 200 = 3$ — The lever makes your push 3 times as strong (its mechanical advantage). 5. $1.5 \div 0.5 = 3$ — Your hand is 3 times as far from the pivot as the rock… 6. $0.1 \times 3 = 0.3 \text{ m}$ — …so to lift the rock 0.1 m, your hand moves 0.3 m. Less force, more distance. Now the pivot is 0.4 m from the rock. Start with the push. **Now the pivot is 0.4 m from the 600 N rock, and you push at the other end. What push balances it, in newtons?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 150 600 × 0.4 = 240, and 240 ÷ 1.6 = 150 N. ::: :::keep-together ### Distance **With the pivot at 0.4 m, how far does your hand move to lift the rock 0.1 m, in metres?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 0.4 1.6 ÷ 0.4 = 4, so your hand moves 4 × 0.1 = 0.4 m. A quarter of the force, four times the distance. ::: :::keep-together ### Find the mistake **Leo thinks the push is 120 N. Click the line where Leo's working first goes wrong.** ``` 1 The pivot is 0.4 m from the 600 N rock. I push at the other end of the 2 m bar. 2 Turning effect of the rock = 600 × 0.4 = 240 3 My push is 2 m from the end of the bar, so push = 240 ÷ 2 = 120 N. 4 120 N is less than 200 N, so I can lift it easily. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 3 Line 3: distances are measured from the pivot, not the end of the bar. The push is 1.6 m from the pivot, so it is 240 ÷ 1.6 = 150 N. ::: :::typical ## Vary: three kinds of lever **A wheelbarrow.** **Your arm.** **Muscle force.** **Why?.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Levers :::keep-together ### Seesaw **A seesaw: a 300 N child sits 1.2 m from the pivot. Where must a 400 N child sit to balance, in metres from the pivot?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 0.9 300 × 1.2 = 360, and 360 ÷ 400 = 0.9 m. ::: :::keep-together ### Crowbar **A crowbar: the load is 0.1 m from the pivot and you push 0.8 m from it. How many times as strong is your push (the mechanical advantage)?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 8 0.8 ÷ 0.1 = 8: your push is 8 times as strong, and your hand moves 8 times as far. ::: :::keep-together ### Three kinds **Where is the pivot? Sort each tool by its kind of lever.** Groups: Class 1: pivot in the middle, Class 2: load in the middle, Class 3: push in the middle - Scissors: {{field-15}} - Crowbar lifting a rock: {{field-15}} - Wheelbarrow: {{field-15}} - Nutcracker: {{field-15}} - Bottle opener: {{field-15}} - Tweezers: {{field-15}} - Fishing rod: {{field-15}} ::: :::teacher-note **Answer:** Scissors → Class 1: pivot in the middle; Crowbar lifting a rock → Class 1: pivot in the middle; Wheelbarrow → Class 2: load in the middle; Nutcracker → Class 2: load in the middle; Bottle opener → Class 2: load in the middle; Tweezers → Class 3: push in the middle; Fishing rod → Class 3: push in the middle Find the pivot first. Then ask which is in the middle: the pivot, the load or the push. Classes 1 and 2 can make your push stronger; class 3 always makes it weaker, but faster. ::: :::keep-together ### Distance **A lever lets a 100 N push lift a 500 N load. The load rises 4 cm. How far does the push move?** ☐ 0.8 cm ☐ 4 cm ☐ 20 cm ☐ 500 cm ::: :::teacher-note **Answer:** 20 cm 5 times the force, so 5 times the distance: 5 × 4 = 20 cm. ::: ## Make: a lever to a brief **Two criteria.** **Evaluate.** Stretch: a real bar has weight. Which side of the pivot would its weight help, and which would it hinder? Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **A 400 N load is 0.25 m from a pivot. You push 1 m from the pivot on the other side. What push balances it, and how far does your hand move to lift the load 0.1 m?** {{lines-3}} ::: :::teacher-note **Answer:** 400 × 0.25 = 100, so 100 ÷ 1 = 100 N; the hand is 4 times as far from the pivot, so it moves 0.4 m. :::