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# Pulleys

Mechanisms · Levers and pulleys

<!-- Speaker note: Students find that one fixed pulley changes only the direction of a pull, add ropes until 150 N lifts a 400 N sack, work out the pull (load ÷ supporting ropes) and the rope pulled (height × ropes), see that work in equals work out, choose a system for a brief and write a brief that needs four ropes. About 45 minutes (Predict and Try 8 min · Worked example 12 min · Vary 10 min · Practise and Make 15 min). Sources: TeachEngineering: K-12 engineering design challenges (benchmark; the briefs and wording are our own), https://www.teachengineering.org/ (checked 12 October 2026). -->

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## Today

**We are learning to** one pulley changes the direction of a pull, not its size. Ropes that hold up a moving block share the load, so the pull shrinks, but you pull more rope by the same factor. Students test pulleys, work out pulls and rope lengths, see that work in equals work out, and choose a pulley system for a brief.

- I can explain that one fixed pulley changes the direction of a pull, not its size.
- I can work out the pull and the rope pulled by counting the ropes that hold the load up.
- I can explain that a pulley system's work in equals its work out.
- I can choose the fewest pulleys that meet a brief.

<!-- Speaker note: Before class: Do Levers first: the Practise match ties gears, levers and pulleys together as one trade. A pair of broom handles and a rope makes a striking demonstration: wrap the rope between them 3 or 4 times and one student can pull two others together. Simplifications: no friction, and the pulleys, blocks and rope weigh nothing. Real systems lose a lot to friction, so they need more ropes than the model says. Curriculum hooks: Design and Technologies, engineering principles and systems (primary); Science, forces and simple machines (Year 7), energy and work (Year 8–9); Maths, multiplication and division. Teacher: Try: ask students to count the rope they pulled before the note appears. Teacher: Worked example: trace each rope with a finger and ask: does this one hold the block up? Teacher: Find the mistake: Mia's fifth rope goes over the top block. Point at where it is tied. Teacher: Vary: link the fixed pulley back to the idler gear: direction, not size. -->

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## Why it matters

> A 400 N sack has to go up onto a stage. You can pull 150 N. How many pulleys do you need, and how much rope will you pull?

<!-- Speaker note: A **pulley** is a wheel with a rope over it. A **block** holds one or more pulleys. In this lesson a **top block** hangs from a beam, a **bottom block** carries the load, and ropes run between them. The rope you pull goes over the top block. The job: lift a **400 N** sack (about 40 kg) **1 m**. You can pull **150 N** at most. -->

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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>

## Predict

**One pulley at the top, and the rope goes from the sack, over the pulley and down to your hand. To lift the 400 N sack you pull…**

- 800 N: the rope goes up and down
- 400 N, but downwards
- Less than 400 N, because the wheel turns
- 200 N: the pulley halves it

<!-- Speaker note: What happens: 400 N. One fixed pulley changes the direction of the pull (you can pull down, using your weight), not its size. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: most students choose 200 N. After the Try, ask why one pulley did not help. -->

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## Try it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fengineering%2Fmechanisms%2Fpulleys%2Fstep-2&code=widget%3A+lever%0Amode%3A+pulley%0Aropes%3A+1%0Achoose%3A+on%0Aload%3A+400%0Apush%3A+150%0Anumbers%3A+off%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/engineering/mechanisms/pulleys/?full=step-2#step-2)

<!-- Speaker note:  -->

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## What happened?

* 400 N. One fixed pulley changes the direction of the pull (you can pull down, using your weight), not its size.

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

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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>

## Four ropes, then six

Now 6 ropes hold up a 600 N load. Start with the pull.

$$4 \text{ ropes hold the bottom block}$$

1) $400 \div 4 = 100 \text{ N}$ — The ropes share the load: each holds 100 N, and so does your hand.
2) $4 \times 1 = 4 \text{ m}$ — To lift the block 1 m, each of the 4 ropes must get 1 m shorter: you pull 4 m.
3) $100 \times 4 = 400 \quad 400 \times 1 = 400$ — Work = force × distance. Work in (100 N × 4 m) equals work out (400 N × 1 m): no free lunch.

<!-- Speaker note: Model this live on the board before revealing anything: write each line and say why, pausing to ask the class what comes next. Then click to reveal the lines and compare. Script. Start: write 4 ropes hold the bottom block and say "Count only the ropes between the blocks. The rope you pull goes over the top, so it does not hold the load up." Line 1: write 400 ÷ 4 = 100 N and say "The ropes share the load: each holds 100 N, and so does your hand." Line 2: write 4 × 1 = 4 m and say "To lift the block 1 m, each of the 4 ropes must get 1 m shorter: you pull 4 m." Line 3: write 100 × 4 = 400 400 × 1 = 400 and say "Work = force × distance. Work in (100 N × 4 m) equals work out (400 N × 1 m): no free lunch." -->

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## Four ropes, then six

**6 ropes hold up the bottom block, and the load is 600 N. What pull lifts it, in newtons?**

* Answer: 100

<!-- Speaker note: 600 ÷ 6 = 100 N. -->

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## Rope

**With those 6 ropes, how much rope do you pull to lift the load 2 m, in metres?**

* Answer: 12

<!-- Speaker note: 6 × 2 = 12 m. Work in: 100 N × 12 m = 1200 J. Work out: 600 N × 2 m = 1200 J. -->

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## Find the mistake

**Mia thinks she needs only 80 N. Click the line where Mia's working first goes wrong.**

```
 1  Mia's block and tackle: 4 ropes hold up the bottom block, and she pulls a fifth rope that goes over the top.
 2  That is 5 ropes, so it shares the 400 N five ways.
 3  Pull = 400 ÷ 5 = 80 N.
 4  To lift the sack 1 m she pulls 5 × 1 = 5 m of rope.
```

* Answer: line 2

<!-- Speaker note: Line 2: the rope she pulls goes over the top block, so it does not hold the bottom block up. Four ropes share the load: 400 ÷ 4 = 100 N, and she pulls 4 m. -->

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## Vary: choose a system



- **Fewest ropes:** 
- **Why one pulley?:** 

[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/mechanisms/pulleys/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Students reason from the brief to the fewest ropes, then connect the fixed pulley back to the idler gear: a mechanism can change direction without changing force. For students who need more support: Give a table with columns ropes, pull, rope pulled for 1 m, work in, and fill the 1-rope and 2-rope rows together. For early finishers: Each pulley wheel loses about 5% of the force to friction. With 4 ropes (and so 4 wheels), roughly what pull lifts the 400 N sack now? The hard part, on purpose: Students count the rope they pull as a supporting rope, and expect any pulley to help. Ask 'does this rope hold the bottom block up?' for every rope. When students get stuck: A student counts the rope they pull. Ask: Does this rope hold the bottom block up, or does it go over the top? Fix: Count only the ropes between the blocks. / A student thinks one pulley halves the pull. Ask: How many ropes hold the sack up? Fix: One rope: the whole load. A fixed pulley only turns the pull round. / A student thinks more ropes means less work. Ask: How much rope did you pull, and with what force? Fix: Pull × rope pulled = load × height, whatever the number of ropes. -->

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## Pull

**3 ropes hold up a 750 N load. What pull lifts it, in newtons?**

* Answer: 250

<!-- Speaker note: 750 ÷ 3 = 250 N. -->

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## Rope

**A crane's hook hangs on 8 ropes. To raise the hook 5 m, how much rope does the winch wind in, in metres?**

* Answer: 40

<!-- Speaker note: 8 × 5 = 40 m. -->

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## Trade-off

**Which uses less rope to lift a 600 N load 2 m: 3 ropes or 6 ropes?**

- They use the same rope
- 3 ropes: 6 m of rope instead of 12 m, but a 200 N pull instead of 100 N
- 6 ropes: more pulleys always means less work
- You cannot tell without the pulley sizes

* Answer: 3 ropes: 6 m of rope instead of 12 m, but a 200 N pull instead of 100 N

<!-- Speaker note: Rope = ropes × height. Fewer ropes: more pull, less rope. -->

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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>

## Same idea everywhere

**Each mechanism trades one thing for another. Match each to what it gives up to make your force bigger.**

| | |
|---|---|
| `Gears, small driving big` | A. Length of rope you pull |
| `Lever, pivot near the load` | B. Distance your hand moves |
| `Block and tackle` | C. Speed of the output |

* Answer: Gears, small driving big → C. Speed of the output; Lever, pivot near the load → B. Distance your hand moves; Block and tackle → A. Length of rope you pull

<!-- Speaker note: All three make your force bigger, and all three make you move farther or faster by the same factor: work in equals work out. -->

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## Make: a brief that needs four ropes



- **Four ropes:** 
- **Evaluate:** Stretch: real pulleys have friction, and the bottom block has weight. Would you need more or fewer ropes in real life? Why?

[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/mechanisms/pulleys/?full=step-6#step-6)

<!-- Speaker note: What this step is for: Writing a brief with a given answer reverses the reasoning: the load ÷ the pull must be more than 3 and no more than 4. Hints, in order: With 4 ropes the pull is load ÷ 4. With 3 ropes it is load ÷ 3. / You need the 3-rope pull to be too big for you, and the 4-rope pull to be small enough. / For example, a 600 N load: 3 ropes need 200 N, 4 ropes need 150 N. Which pull would you choose? -->

<!-- Speaker note: Teacher: compare briefs; whose load ÷ pull ratio is closest to 4? -->

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## Exit ticket

**5 ropes hold up a 1000 N load. What pull lifts it, and how much rope do you pull to lift it 2 m?**

* 1000 ÷ 5 = 200 N, and 5 × 2 = 10 m.

<!-- 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: the Capstone projects (Physical computing) include a lift and a winch where these trade-offs meet a motor. Real examples: cranes, sailing boats (a mainsheet), flagpoles, window blinds, gym machines, rescue hauling systems. -->