Atwood's machine is the name of a device that looks like this: Also known as "two masses on a pulley". Surprisingly, this simple device comes up a lot in intro physics texts.

With a simple pulley the force you must exert is equal to the weight of the load. With a compound pulley—two or more pulleys combined together—you can reduce the force required to lift the load.

EXPLANATION: A pulley is one of the six simple machines. Single pulleys were part of simple cranes invented more than 3,000 years ago. A pulley consists of a wheel that is free to turn its center.

Explore the clever mechanics behind pulleys in Jiwi's Machines Episode 2. Discover how simple systems create powerful movements in this exciting Science Extra feature.