A pulley is one of the simplest machines, essentially a circular lever in the form of a wheel or fixed curved block, with a groove around it to accommodate a rope or belt. The earliest evidence for the existence of the pulley comes from Assyria in the eighth century B.C.E. a painting of a battle scene shows a warrior using a simple pulley to lift a bucket over a wall.
Pulleys are mainly used to move or lift a load. A single fixed pulley can be used to change the direction in which a force is applied, as it may be easier to pull on a rope than to drag or push the load. When the rope is fixed at one end and another pulley is added, the system effectively halves the required force, as each part of the rope carries an equal share of the load. This does not reduce the mechanical work required: work is the multiple of force and distance and the rope—now doubled in length—will need to be pulled twice as far. More pulleys can be added to make a "compound pulley" system, further multiplying the effectiveness of the force applied.
Pulleys have been in use throughout the world for many centuries. Although the earliest hard evidence of their use dates from the eighth century B.C.E., it is highly likely that the principle was in use long before that. Early humans most likely created pulleys by throwing a rope or a fibrous vine over the branch of a tree to hoist up a heavy weight.
It is highly probable that pulleys were used by the builders of early massive constructions, such as the ziggurats of Mesopotamia (built as early as the fourth millennium B.C.E.), Stonehenge (built in 2200 B.C.E.), and the pyramids of Egypt (third millennium B.C.E.).
