How Does a Pulley System Work?

A pulley consists of a rope or line that is run around a wheel. A pulley allows force to be directed in a downward motion to lift an object. Say you had a ten-pound object that you needed to move 50 feet in the air.

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. Now, if you use a pulley, it makes this work much easier.

You’re still having to apply the same amount of force, but you are 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.

This rope is 50 feet and there’s a person down here pulling on the rope. The load is moved the same distance as the rope pulling it. Actually, this rope might be 100 feet long or 75 feet long. In this case, what this means is that the distance between the top of the pulley and the object you’re moving is 50 feet here.

You’re going to have to pull that rope 50 feet to get the object up here. That’s a very simple concept to understand. Say you used a combination pulley, such as a double pulley. The weight is moved half the distance of the rope pulling it.

To explain that, I’m going to have to draw a diagram. What we have here is a pulley with a ten pound object on it. We have another pulley up here and 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 10 pounds. Over here, the work was not necessarily made any easier, because you still had to apply 10 pounds of force. Here, a big 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 when I said a second ago: the weight is moved half the distance of the rope pulling it.

Say this rope right here is 50 feet. You want to move this triangle all the way up here. Well, you have to pull the rope more than 50 feet. Say you pulled the rope ten 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.

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 100 feet. The work here really isn’t reduced, because you can’t reduce the amount of work.

Work is moving an object over a distance. You could only reframe that work to make it easier. 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. You may be thinking, “Does that really make it any easier?”

With a ten-pound weight you’re trying to move, not really, but try to think of this in a different sense. Say this weighed 200 pounds. You may not be able to move a 200-pound weight at all. Say you only have to apply 100 pounds of force now.

You’re gonna have to do it twice as much, but that’s a lot better, because you weren’t able to even move- you weren’t you weren’t even able to budge the 200-pound weight. Now you have a 100-pound weight, which is more manageable.

You can actually move that weight. Yes, a 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 lessen the amount of work but it just makes it easier for the person doing that work.



by Mometrix Test Preparation | Last Updated: January 14, 2021