This is a pulley, pulled straight off Wikipedia:
On the test that my students had last week, there was a question involving pulleys connected in a sort of eclectic arrangement. The objective was to find the tension in certain ropes which were holding the pulleys up.
Imagine instead of being connected to the ceiling, the pulley in the picture is being held up by your hand. Hanging from the pulley is a heavy metal block, and the rope holding the block up is itself being temp taut by another person. So just like the picture, except for simplicity assume that the rope is not pulled at an angle as in the picture but instead has both ends pointing directly down.
And let’s say the weight weighs 10 pounds. How much force do you have to apply to the pulley to hold it up?
The answer is 20 pounds. Lots of people got the equivalent of this question wrong. Some of my students too, I am sad to say. We never directly discussed pulleys so I suppose it’s not a shock. The reason for the 20 pounds is this: there are two forces acting on the pulley: the weight down, and the person pulling the other end down. The person pulling the other end will clearly have to be applying 10 pounds to balance the weight, so the sum of those forces is 20 pounds. If it makes things any clearer, you can imagine that the person holding the rope could be replaced by another 10 pound weight.
So if you’re ever using a pulley to haul buckets of concrete up to the roof, keep that in mind: a system like this has to support more than just the weight of the concrete load.