Pulleys are wheels with grooves through which a rope can run. It is much easier to pull something down than to lift it up, as you can use your body weight to help. A pulley enables a downward force to be converted into an upward force. Lifts often use this principle by employing a counterweight. The dropping down of the lift causes the counterweight to rise. Then the dropping of the counterweight helps the lift to go up again.
One wheel
If you have a single wheel and a rope, a pulley helps you reverse the direction of your lifting force. So, you pull the rope down to lift the weight up. If you want to lift something that weighs 100kg, you have to pull down with a force equivalent to 100kg, which is 1000N (newtons). If you want to raise the weight 1m into the air, you have to pull the loose end of the rope a total distance of 1m at the other end.
Two wheels
Now if you add more wheels, and loop the rope around them, you can reduce the effort you need to lift the weight. Suppose you have two wheels and a rope looped around them, The 100kg mass (1000 newton weight) is now effectively supported by two sections of the same rope (the two strands) instead of just one (ignoring the loose end of the rope you’re pulling with), and this means you can lift it by pulling with a force of just 500 newtons—half as much! That’s why we say a pulley with two wheels, and the rope wrapped around it this way, gives a mechanical advantage (ME) of two.
Mechanical advantage is a measurement of how much a simple machine multiples a force. The bigger the mechanical advantage, the less force you need, but the greater the distance you have to use that force. The weight rises 1m, but now we have to pull the loose end of the rope twice as far (2m). How come? To make the weight rise 1m, you have to make the two sections of rope supporting it rise by 1m each. To do that, you have to pull the loose end of the rope 2m. Notices that we can also figure out the mechanical advantage by dividing the distance we have to pull the rope by the distance the weight moves.
Picture Credit : Google