A pulley is one of the oldest and simplest machines - just a wheel with a groove for a rope. Pull down on one side, and a weight on the other side goes up. By itself, that doesn’t make lifting any easier. It just changes the direction of the pull, which is handy when you’d rather pull down than lift up.
The magic happens when you combine pulleys. A “block and tackle” loops one rope through several pulleys at once. With two wheels, you only need half the force to lift a weight. With four wheels, a quarter of the force. With eight, an eighth. The trade-off is that you have to pull the rope much farther.
Cranes, elevators, sailboats, theater curtains and even the cable cars in San Francisco all use pulley systems. The ancient Greek inventor Archimedes was so impressed by them he reportedly said “Give me a place to stand and I will move the Earth.”
