A pulley consists of a rope or line that is run by a wheel. A pulley allows the force to be directed in a downward motion to lift an object.

Say you had a 10-pound object that you needed to move 50 feet in the air, where you’re going to have to attach a rope to it and pull that rope in an upward motion 50 feet to get that object suspended 50 feet in the air.

The transcript is for your convenience.

Now if you use a pulley, it makes your work much easier. You’re still going to have to apply the same amount of force, but you’re able to apply that force in a downward direction. We have a pulley right here, we have a rope around it, and then we have this triangle that weighs 10 pounds and then this rope is 50 feet. Then there’s a person down here pulling on the rope, and so the load is moved the same distance as the rope pulling it.

Now actually this rope might be a 100 feet long or 75 feet long, well in this case what this means is, is that the distance between the top of the pulley and the objects you’re moving is 50 feet here. You’re going to have to pull that rope 50 feet to get the object up here, so that’s a very simple concept to understand.

Now, say you used a combination pulley, such as a double pulley, the way it’s moved half the distance of the rope pulling it. Now to explain that I’m going to have to draw a diagram, so basically what we have here is a pulley here with a 10-pound object on it, and we have another pulley up here then the force is being applied right here. This pulley stays in place kind of like this pulley, but this pulley moves around with the object.

This triangle here is still ten pounds. Over here, work was not necessarily made any easier, because you still had to apply 10 pounds of force. Now here the difference is made since we’re using a double pulley. You only have to apply five pounds of force, because the other five pounds is being held up over here next to the ceiling or the tree or whatever this is over here.

Now you’re thinking the work is a lot easier, but remember what I said a second ago, the way this moved half the distance of the rope pulling it, okay. Say this rope is 50 feet, so you want to move this triangle all the way to up here, well you have to pull the rope more than 50 feet. Because say you pull the rope 10 feet, well five of those feet would be used to reduce the distance right here, but five of those feet would be used to reduce the distance right here and so as that happens the pulley is going to move, up and up and up.

Really if you want to move this triangle 50 feet, you’re going to have to pull the rope a 100 feet and so the work here really isn’t reduced, because you cant reduce the amount of work, because work is moving an object over a distance. You can only refrain that work to make it easier, so here you don’t have to apply as much force, you only have to apply half as much force, but you have to apply it for twice as long.

Now you may be thinking that’s not really making it easier, well with a 10 pounds weight you’re trying to move not really. Try to think of this in a second sense, say this weighed 200 pounds, you may not be able to move a 200 pound weight at all, but say you have to apply 100 pounds of force now. Now you’re going to have to do it twice as much now, but that’s a lot better, because you weren’t able to budge the 200 pound weight, but now you have a 100 weight which is more manageable and so you can actually move that weight. Yes double pulley, definitely does come in handy. Remember again that a pulley uses this concept of mechanical advantage to make work easier, because it changes the way you’re doing work. It doesn’t necessarily less than the amount of work, but it just makes it easier for the person doing that work.