A pulley is a simple machine that is the basis of much modern machinery. This machine changes the direction of an applied force, allowing a heavy load to be moved. A person can pull on the cable part of the device, and it will pull a weight. There are six simple machines: an inclined plane, a lever, a screw, a wedge, a wheel and axle, and a pulley.
At its most simple, a pulley is a wheel with a groove. A rope, belt, or cable runs inside the groove. That mechanism can be used alone or connected with others in a pulley system. The greater the number of pulleys in the system, the less force it will take to lift an object.
A block and tackle system, which is a basic system, can illustrate how this machine provides this kind of lifting advantage. A 10-pound (4.5-kg) weight has a rope attached to it. To lift the weight, a person will need to exert 10 pounds of force (4.5 kilogram-force) straight up. If the weight is attached to one pulley, the person still needs to exert 10 pounds of force (4.5 kgf), but the direction it needs to be exerted in is down, instead of up, making use of the individual's body weight more efficiently. If the weight is attached to two pulleys, the person will need to exert only 5 pounds of force (2.26 kgf). Four pulleys will require 2.5 pounds (1.133 kgf) of force, and so on.
There are three basic types of pulley systems: (1) fixed, with a fixed axle, (2) movable, with a free axle, and (3) compound, with a combination of fixed and movable axles. Movable and compound axles, by multiplying the force, are more efficient than fixed systems.
Another type of system is the belt and pulley system, which can often be seen under the hood of a car. These systems are closed systems in which several pulleys are connected by one belt. They allow for the transmission and amplification of energy from one pulley to another.