I genuinely don't understand how this works. It makes no intuitive sense to me.
If I'd look at this I'd think you'd have to pull 100N since you not only have to lift the weight but also got friction on the rolling parts.
Someone explain this magic to me. I'm a fucking retard.
Anything with numbers like this doesn't include friction. The weight is split in half. 50 going to the ceiling through the purple part of the pulley, and the other 50 going to the long part of the rope. All the pulley does is change the direction of the force. It changes the red 50 so that it pulls up on the weight
I don't know what FBD is, sry. I don't usually frequent this board. I just thought maybe someone can explain this to me in simple terms.
It's such a great invention but I don't get it.
nvm I found a decent pic I think.
You only move it half the distance each pull since you're basically just lifting one side of the load-bearing rope, right?
It distributes the "work" over a greater area; Think of a long lever with the fulcrum near the load you're lifting -- you have to move the end of the lever a long way in order to move the heavy load just a little bit. That's how a block & tackle works.
The top pulley must support the weight as well as the downward tug on the rope, which totals to 150 Newtons. Since the leftward rope between the two pulleys is supporting half the weight of the metal weight, the other half is transferred over the top pully to the force tugging on the rope. However, the work-energy theorem states that you will have to pull twice the distance to achieve the same change in height.
Think of it in terms of energy/work. W = Fd. Since there are two lengths of rope supporting the mass, you need to shorten each by the length d in order to raise the mass by that much. Combined, that makes 2d, which is how far you have to pull the rope.
F(pull) * 2d = F(mass) * d
Basically it's like a lever. You're pulling farther, so you don't have to pull as hard.
Oh, but on second thought I see what you mean... shortening the rope supporting the weight by a particular distance doesn't raise the weight by that distance... so there's a trigonometric relationship there.