Wikijunior:How Things Work/Pulley
A pulley is a wheel with a groove along its edge, that holds a rope or cable. Usually, two or more pulleys are used together. When pulleys are used together in this way, they reduce the amount of force needed to lift a load. A crane uses pulleys to help it lift heavy loads. Pulleys are one of the six simple machines.
Who invented it?
The origin of pulleys is not known, but there is evidence that they were used in the building of Stonehenge, in England. One of the first names we associate with pulleys is that of Archimedes, a famous Greek scientist born in Syracuse (a Greek seaport colony). Archimedes studied many areas of science and invented many devices, and it is said that he used pulleys to drag a ship out of the ocean and onto land. Archimedes did not hold public office but devoted his entire lifetime to research and experiment. He was educated at Euclid's school in Alexandria, a major world city of the time, located in modern-day Egypt. Many of Archimedes' experiments impact on our daily lives today...
How does it get power?
It gets its power by trading distance for effort. With a 2 pulley system, you can use half as much force to lift something, but you have to pull the rope two meters to lift an object one meter.
Why does that work? It's actually because of a concept called work. You know that if you walk 2 miles (3km) to school, it's harder than if you had to walk 1 mile to school. How much effort does each step take? The effort is however much it takes to move your body. Scientists and engineers call how much effort you use over a distance work. A real rough equation is to say (this isn't exactly true). So, if you pull the rope in a 2 pulley system 2-ft (60cm) (. with 1-lb force, then you did 2-ftlbs. (read "foot pounds") of work, which is the same amount of work as pulling just a rope 1-ft. with 2-lb force. One consequence of the 1st Law of Thermodynamics is doing the same process (like lifting a box 1 ft) takes the same amount of work, no matter what. So you can use any force you want, as long as you do the same amount of work.
How does it work?
A pulley is a rope wrapped around a wheel. It changes the direction of force. A basic compound pulley has a rope attached to a stationary point looped around one wheel and then around a second wheel. Pulling on the rope pulls the two wheels closer together.
How dangerous is it?
Care must be taken to avoid getting fingers, loose clothing, and hair caught in its workings. Also since pulleys are often used to lift heavy objects one should be careful. If the rope, cable, or hardware which mounts a pulley were to fail, serious injury could result if you happen to be under the load. One must be careful not to exceed the documented rating of lifting equipment.
What does it do?
It is used to pull two things closer together. It can be used to lift heavy objects (by pulling the heavy object closer to something above). It is sometimes used as a trap. the person would hook a net to the pulley's rope and then the person would wait until what he wanted to catch is caught.
How does it vary?
You can add or subtract pulleys from the system. More pulleys makes it easier to pull (or they allow you to pull a heavier load), but you have to pull more rope to move the load by the same amount.
Many different types of pulley systems exist. For an example, the average sailboat may have 3 or 4 different pulley systems (boomvang, cunningham, outhaul, mainsheet), all doing different things.
How has it changed the world?
Pulleys allow people to pull heavier objects around than they could using only their muscles. This allowed ancient countries to build large sailing ships and explore the world, because people would not be able to pull the sails into the right place without them. Pulleys can also pull large stone blocks upward, and so they help people to build large buildings.
What ideas and/or inventions had to be developed before it could be created?
Rope and the wheel.