A pulley is a grooved wheel with a rope or cable moving around the groove
Perhaps the best known use of this simple machine is using it to hoist a flag up a flagpole.
Pulling down on the rope makes the flag rise up the pole.
You now have the advantage of not having to climb the pole every time you raise or lower the flag but this does not offer you what is known as mechanical advantage.
Mechanical advantage is the ratio of input force to the output force applied to an object.
The pulley does not change the amount of force that is applied. Here, if the flag weighed 50 pounds, you would have to pull 50 pounds downward to raise the flag upward.
Now let's look at instances in which a pulley can be used to offer a mechanical advantage.
Let's suppose you wanted to put shingles on a roof and you need to haul up the shingles.
You could secure a rope to the crate, climb the ladder while you carry the rope up and then raise the shingles up to the roof. The problem is that the crate weighs 200 pounds and that's far too much weight than you can lift!
(Yes, the shingles could be organized into several smaller batches, but you know your high school physics and you would like to do this in just one attempt.)
So, you get a pulley and get twice the length of rope you needed for the "unassisted lift".
You thread the rope through the pulley, secure the pulley to the crate, then you take both ends of the rope up the ladder with you.
When you reach the roof, secure one end of the rope to the edge of the roof (point A) and then lift the crate with the other end of the rope.
It's much easier this way isn't it? Now you only have to lift 100 pounds.
Looking at the diagram, the weight of the shingles is now being split between the 2 ropes.
We said before that simple machines do not give you something for nothing. Here, you now only have to lift 100 pounds but it requires moving the shingles twice the distance.
Still, lifting 100 pounds of shingles up to a roof is no easy task is it?
Let's try using the high school physics again.
We could get another pulley and secure that to the roof. (point B)
Do you notice that now you can no longer lift the shingles to the roof from the roof anymore?
Adding another pulley has reversed the direction of the force.
Basically, in order to lift the shingles to the roof, you have to be on the ground.
Speaking very unscientifically this does save you a tremendous amount of work.
You get a friend to stay on the ground, pull down on the rope and lift the shingles up to you!
Yes, getting someone else to do the "heavy lifting" is quite an advantage but it is not, scientifically speaking, a mechanical advantage.
So, let's try another approach.
We get a third pulley, secure it to the crate, secure it to the other pulley with an iron bar and make sure we use four times the length of rope we needed in the single pulley system.
Looking at the diagram, we see that the weight of the shingle crate is hanging from 4 ropes. Basically, 200 pounds of weight is being distributed among the four ropes, and each rope has 50 pounds acting on it. One of those ropes is the one you are using to lift the shingles and you now only need 50 pounds of force to raise 200 pounds of shingles to the roof!
(Of course you must remember we had to move these 4 times the distance ... but they made it to the roof!)
When the effort is attached to a moving pulley and when the effort moves in the same direction as the load, the pulley system is said to be rove to advantage. (Diagrams 1 and 3)
When the effort is attached to a fixed pulley and when the effort moves in an opposite direction to the load, the pulley system is said to be rove to disadvantage. (Diagrams 2 and the flagpole diagram)