Pulleys

Classroom resources

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Students will be able to

• 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.

Timing

Predict and Try 8 min · Worked example 12 min · Vary 10 min · Practise and Make 15 min

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.

Running the lesson

1. Predict: most students choose 200 N. After the Try, ask why one pulley did not help.
2. Try: ask students to count the rope they pulled before the note appears.
3. Worked example: trace each rope with a finger and ask: does this one hold the block up?
4. Find the mistake: Mia's fifth rope goes over the top block. Point at where it is tied.
5. Vary: link the fixed pulley back to the idler gear: direction, not size.
6. Make: compare briefs; whose load ÷ pull ratio is closest to 4?

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

Answers to the checks

1. one pulley: 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…
   400 N. One fixed pulley changes the direction of the pull (you can pull down, using your weight), not its size.
2. One rope: With one rope holding the sack, press Pull.
   
   It does not move: one rope holds the whole 400 N, and you can pull only 150 N.
3. Add ropes: Add ropes (more pulleys) until your 150 N lifts the sack. Use as few as you can.
   
   Three ropes: each holds 400 ÷ 3 ≈ 133 N, and 133 N is less than 150 N. Two ropes would each need 200 N.
4. Rope pulled: With 3 ropes, how much rope did you pull to lift the sack 1 m (in metres)?
   3
   3 m: each of the three ropes holding the block shortens by 1 m, and all of that rope comes out through your hands.
5. Four ropes, then six: 6 ropes hold up the bottom block, and the load is 600 N. What pull lifts it, in newtons?
   100
   600 ÷ 6 = 100 N.
6. Rope: With those 6 ropes, how much rope do you pull to lift the load 2 m, in metres?
   12
   6 × 2 = 12 m. Work in: 100 N × 12 m = 1200 J. Work out: 600 N × 2 m = 1200 J.
7. Find the mistake: Mia thinks she needs only 80 N. Click the line where Mia's working first goes wrong.
   line 2
   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.
8. Fewest ropes: A 900 N engine, and you can pull 200 N. Lift it with as few ropes as you can (fewer ropes means less rope to pull).
   
   900 ÷ 200 = 4.5, so 5 ropes: 180 N each. 4 ropes would need 225 N.
9. Why one pulley?: If one fixed pulley does not make the pull smaller, why use one?
   It changes the direction: you can pull down, using your own weight, instead of lifting up
   Like an idler gear, a fixed pulley changes the direction, not the size. Flagpoles, curtains, and the top pulley of every block and tackle do this.
10. Pull: 3 ropes hold up a 750 N load. What pull lifts it, in newtons?
    250
    750 ÷ 3 = 250 N.
11. 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?
    40
    8 × 5 = 40 m.
12. Trade-off: Which uses less rope to lift a 600 N load 2 m: 3 ropes or 6 ropes?
    3 ropes: 6 m of rope instead of 12 m, but a 200 N pull instead of 100 N
    Rope = ropes × height. Fewer ropes: more pull, less rope.
13. 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 → C. Speed of the output; Lever, pivot near the load → B. Distance your hand moves; Block and tackle → A. Length of rope you pull
    All three make your force bigger, and all three make you move farther or faster by the same factor: work in equals work out.
14. Four ropes: Set a load and your pull so that 4 ropes is the fewest that works. Then choose 4 ropes and lift.
    
    The load ÷ your pull is more than 3 and no more than 4: 4 ropes is the fewest that works.

Solutions

One possible solution for each task. Other correct programs are fine.

Vary: choose a system › Fewest ropes

Open task: any program that does what the step asks.

Vary: choose a system › Why one pulley?

Open task: any program that does what the step asks.

a brief that needs four ropes › Four ropes

Open task: any program that does what the step asks.

a brief that needs four ropes › Evaluate

Open task: any program that does what the step asks.

Stretch and support

Stretch: 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?

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.

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.

What each step is for

• one pulley: Many students expect any pulley to make lifting easier. Committing first makes the result stick: a single fixed pulley only changes direction, like an idler gear. Each wrong option is a real slip: halving, or the turning wheel helping.
• Try: add ropes: One rope fails at 150 N; adding ropes that hold the bottom block until it lifts settles the size question, and the rope pulled is reported, so students meet the cost before the rule.
• Worked example: count the ropes: The worked example gives two rules from one count: pull = load ÷ supporting ropes, rope pulled = height × supporting ropes, and shows work in = work out. The faded example uses 6 ropes. Mia counts the rope she pulls, planted after the method.
• Vary: choose a system: 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.
• pulleys: Count the supporting ropes, divide for the pull, multiply for the rope, and the same trade as levers and gears.
• a brief that needs four ropes: Writing a brief with a given answer reverses the reasoning: the load ÷ the pull must be more than 3 and no more than 4.

Where it 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.

Sources

• TeachEngineering: K-12 engineering design challenges (benchmark; the briefs and wording are our own) (checked 12 October 2026)