A pulley is a simple machine made of a wheel with a groove for a rope. It changes the direction of a force, making it easier to lift heavy things. A single pulley lets you pull down on a rope to lift something up. When you combine multiple pulleys, you can reduce the amount of force needed to lift a load, though you have to pull the rope a longer distance.
Set up a simple pulley with a spool, string, and a small bucket of weights. Let students feel the difference between lifting the bucket directly and lifting it through a pulley. Add a second pulley to make a compound system and compare the effort needed.
Have you ever watched a flag being raised on a flagpole? You pull the rope down, and the flag goes up. That is a pulley at work. A pulley is a wheel with a groove around its edge where a rope or cable sits. When you pull one end of the rope, the other end moves in the opposite direction. This simple setup changes the direction of your force, which is incredibly useful.
A single pulley attached to a fixed point — like the top of a flagpole — is called a fixed pulley. It does not actually reduce the force you need; a 10-pound flag still takes about 10 pounds of pull to raise. What it does is let you pull downward instead of lifting upward, which is much more natural since you can use your body weight to help.
Things get really interesting when you combine pulleys. A compound pulley system uses two or more pulleys together. In these setups, the rope wraps around multiple wheels, and each section of rope shares part of the load. With two pulleys, each rope section only carries half the weight, so you pull with half the force. With four pulleys, you only need one-quarter of the force. The catch? You have to pull the rope much farther — four times as far in a four-pulley system.
Pulleys are hidden all around you. The cables inside an elevator run through pulleys. Construction cranes use compound pulley systems to lift steel beams that weigh thousands of pounds. Exercise machines at gyms use pulleys to redirect the force of weights. Even the blinds on a window use tiny pulleys. Wherever there is a heavy load to lift or a force that needs to change direction, pulleys are one of the simplest and smartest solutions.