A closer look at pulleys: single vs double
Physics · Key Stage 2 · The School
A pulley changes direction
A single, fixed PULLEY is just a wheel with a groove. It lets a rope run over it. Pulling DOWN on one end lifts a load UP on the other. The pulley has changed the DIRECTION of the pull. That is genuinely useful. Pulling down, using your own body weight, is often easier than pulling up. But the load is not any lighter.
Adding a second pulley
Add a second, moving pulley, attached to the load itself. Something new happens. The load can now be lifted using roughly HALF the force. The weight is shared between two lengths of rope instead of one. There is a trade-off, though. You must pull the rope roughly twice as far, to lift the load the same height.
The trade that never disappears
A pulley system can reduce the FORCE needed. But the total WORK stays roughly the same. Work means force multiplied by distance, once friction is ignored. Pulling with half the force over twice the distance is not free lifting. It is exactly the same job. It is simply spread out differently, made to feel easier.
A 20 kg crate is lifted with a single fixed pulley. The full weight is felt on the rope. But pulling down feels natural, using body weight. The same crate is then lifted with a two-pulley system instead. The rope now needs only about half the pulling force. But it must be pulled through about double the length of rope, to raise the crate the same height. Neither method makes the crate lighter. Neither makes the job free. One changes the direction of effort. The other trades force for distance.