Pulleys
How one rope can lift 4 times your pull.
2 · The question
A 40 kg crate hangs from a rope that runs round some wheels. Could you hold it up with a pull like 10 kg?
3 · Play first
Pick a pulley. Lift the crate. Count the ropes.
Drag the orange pull arrow down to lift the crate, or tap any part to see what it does.
- Pull needed
- 98 N (like 10.0 kg)
- Crate rises
- 0.00 m
- Rope you pull
- 0.00 m
4 rope parts share the crate: a 1/4 of the pull, but 4 m of rope for every metre it rises.
4 · What's happening
- 1
A wheel on a hook only turns your pull. You pull down, the load goes up, with the same force.
- 2
Hang a wheel on the load and two rope parts hold it up. Each one carries half.
- 3
Count the rope parts holding the moving block. That is how many times easier it gets.
- 4
The catch: you pull that many metres of rope for every metre the load rises.
5 · The maths
The pull is the load's weight shared between the rope parts holding it. The rope you pull grows by the same number.
F = 392 N ÷ 4=98 N
rope = 4 × 0.00 m=0.00 m
work in 0 J=work out 0 J
Less force, more rope: the work you put in is the work the crate gets.
6 · Under the hood
Tackles, friction and a famous ship (for grown-ups)
Sailors name them by the count: gun tackle 2, luff tackle 3, double tackle 4, gyn tackle 5, threefold purchase 6.
Every wheel rubs on its axle, so each one you add helps a little less. Past a point, more wheels stop helping.
Sailing ships (where pulleys are called blocks), cranes, drilling rigs and oil derricks, and tree-care rigging.
Plutarch says Archimedes, sitting far off, drew a loaded three-masted merchant ship towards him with a compound pulley. It had been hauled ashore by many hands.
7 · Try it yourself
Pulley hunt
Find 3 pulleys around you, on a crane, a sailing boat or a picture of one. Tap the button each time you find one.
Build it
The pulley kit is on its way: wheels and rope you can thread yourself.
Tell me when it's ready