A pulley is simply a collection of one or more wheels over which we loop a rope to make it easier to lift things. A pulley system makes it easier to lift an object than lifting the dead weight by hand. A single pulley essentially changes the direction of the pull or force applied. When a person uses two or more pulleys in a system, then the system also multiplies the force applied besides changing its direction.

Fig: Simple Pulley Diagram:

A Pulley is one of the simple machines. The original primary use for pulleys was to make it easier to lift heavy items. A pulley is a simple machine made with a wheel and a rope, cord, or chain. Pulleys can be found everywhere, helping us in making our tasks easier.

A pulley doesn't use fuel or electricity, and it doesn't run on its own, but it's still a machine. Pulleys can be made from a variety of materials, including an extensive range of plastics, wood and metals. Steels and aluminum alloys are regularly used in industrial pulley manufacture; many pulley designs incorporate multiple materials in order to wed strength with proper resistance characteristics.

Working of Pulley:

As shown in above figure if we have single wheel and a rope, a pulley helps us to reverse the direction of our lifting force. So, as in the picture above, we pull the rope down to lift the weight up. If we want to lift something that weighs 100kg, we have to pull down with a force equivalent to 100kg, which is 1000N (newtons). If we want to raise the weight 1m into the air, we have to pull the loose end of the rope a total distance of 1m at the other end.

If we add more wheels, and loop the rope around them, we can reduce the effort you need to lift the weight. Suppose we have two wheels and a rope looped around them, as in the figure below. The 100kg mass (1000 newton weight) is now effectively supported by two sections of the same rope instead of just one , and this means we can lift it by pulling with a force of just 500 newtons—which is nearly half.

Mechanical advantage is a measurement of how much a simple machine multiples a force. The bigger the mechanical advantage, the less force we need, but the greater the distance we have to use that force. Hence pulley uses same mechanical advantages.