These blue plastic pulleys have are mounted on aluminum metal frames using screws and nuts so that you can disassemble the and re-purpose them as needed. Each pulley has a hook on one end.
Pulleys are often used as parts of compound machines. They function as helper machines, easing the lifting power. In fact, a pulley system can cut the force necessary to lift a very heavy object in two, four, etc., depending on the designed system. With these pulleys you can study and measure the force and distance trade off with compound machines. These pulleys are good for science experiments as well as functional tasks.