A pulley is a set of wheels and rope that allows you to move heavy objects with minimal effort. Pulleys are one of humanity's first inventions and are important parts of machines we use every day. Cranes at construction sites use complex pulleys called block-and-tackle systems to lift extremely heavy loads long distances above the ground, while elevators use pulleys to lift people up many floors in tall buildings.
Force is the effort used to move something or to stop something from moving. A pulley allows us to change the direction of the force while multiplying the amount of force applied to the load. It can also simply ease the mechanics of lifting a load. For example, imagine dropping a one-gallon bucket into a well. Because a gallon of water weighs eight pounds, it takes eight pounds of force to lift that bucket. By using a simple one-wheel pulley, a person can pull down on the rope instead of up, which many people find easier because of the increased leverage. This pulley does not amplify force.
The most important use of pulleys is to magnify force, which allows heavier loads to be lifted with less force than otherwise. The trade-off is that the distance the rope has to travel is increased proportionally. A complex pulley that uses more than one wheel amplifies the force of the lifter's pull, but the person has to pull farther. In the well example, a two-wheel pulley will allow the lifter to bring up a two-gallon bucket with the same amount of energy that a one-wheel pulley needs to lift a single gallon. The rope, however, must travel twice as far.
Pulleys are all around us. Simple pulleys are used in elevators, escalators and some construction site cranes. These simple pulleys need only change the direction in which the force is applied. When heavy things need to be lifted or moved, complex pulleys are used to magnify the lifting power. These complex pulleys are usually referred to as in "block and tackle." Pulleys are not only used to lift things. They can be useful when a lot of force is needed to pull something, such as a tree stump, from the ground.
Elevators use simple pulleys to lift people up and cranes at construction sites use pulleys to lift heavy loads such as steel girders to construct buildings. In an elevator, a single pulley is used. On one end of the cable is the elevator car, on the other end a heavy counterweight equal to the heaviest load the elevator can carry. Brakes use friction to slow the elevator when the weight of the car is less than the counterweight.
Certain types of cranes, called tower cranes, use a block-and-tackle mechanism to lift objects high into the air during the construction of skyscrapers. Block and tackle uses several pulleys linked together in parallel lines, one above the other. A strong cable is wound between the two lines of pulleys starting at the motor, provides the power to do the lifting and, after threading through the pulley systems, is attached to the support that holds the axle of the top line. While the top line of pulleys is attached to the arm of the crane, the load is attached to the axle of the lower set. As the motor winds the cable, the lower axle is pulled toward the upper axle, lifting the load. The force is increased by the number of pulleys in both systems.