A doorknob, a steering wheel and a screwdriver handle all look different, but they work the same way: a big wheel turns a small axle.
Key idea
In a wheel and axle, a big wheel and a small rod (the axle) turn together. Turning the big wheel takes less force to turn the axle. A pulley is a wheel with a rope in its groove. A fixed pulley changes the direction of your pull. A movable pulley halves the pull you need.
| Pulley setup | To lift 20 pounds you pull about | Rope you pull for each foot of lift |
|---|---|---|
| One fixed pulley | 20 pounds (but you pull down) | 1 foot |
| One movable pulley | 10 pounds | 2 feet |
| Two ropes each holding half, plus more pulleys | less again | more again |
(Real pulleys have a little friction, so you pull a bit more than this.)
Worked example: Raising a flag
Why does a flagpole have a pulley at the top?
- You cannot climb up to lift the flag.
- The fixed pulley lets you pull the rope down at the bottom to raise the flag up.
Answer: It changes the direction of the pull.
Common mistake
Thinking a single fixed pulley makes things lighter. It only lets you pull in an easier direction.
Try it at home
- Hang a broom handle between two chairs. Tie a rope to a bag with a few books in it.
- Loop the rope over the broom handle and pull down to lift the bag. You made a simple fixed pulley (with lots of friction).