I asked this at rsurvival earlier today to comeback and see a mod removed it. Wild. Aren't pullies an important homesteading or survival tool?
Assuming this question is relevant to this sub hears what I'm wondering.
What's the load on the rope in the
section where it rounds the pully.
Here's a picture to help explain.
Logic says each section of rope going up from pullie A has a 50 kg load on it and that the section of rope that rounds Pullie A is under a load
more than 50 kg and something less than 100 due
to some physics magic about rope and circles and
friction I don't understand. Anyone want to enlighten
me?
by levelshevel
11 Comments
The two ropes attached to the weight split the load. If you doubled it up and had 4 ropes the load would be split 4 ways.
I’m not quite understanding what you’re struggling with since you attached this diagram.
The orange dots in the diagram are fixed points if that helps.
Maybe you could look up free body diagrams to get a better answer that you could understand?
Also, pully.
Pulley A (The Load) will move up 1 inch when you pull the rope 2 inches. So, the force you pull is half, but you pull twice as far (mechanical advantage of 2).
The top pully only changes direction; it has no impact on mechanical advantage. The only pullys that will give a mechanical advantage are those that are moving
A couple things to say: 1. A pulley basically exchanges distance for force. You’ll pull that rope 2x as far as the weight will move, but you’ll exert 1/2 of the force needed to lift it directly. 2. The load at any point on that rope is 50kg. Think about it this way, the mass has two lengths of that rope each pulling up with 50kg of load, and the mass is pulling down with 100kg of load. They balance perfectly.
Because at the apex of where the rope rounds the pulley the pulley, if you were to fix it in space there would be a 490 newton (50kg) force acting in both directions. It’s not totally fixed though and so instead the minimal amount of friction and resistance from the pulley adds a force resisting the direction of movement. There’s also a total 980 newton (100kg) shear force occurring to account for but I’m not entirely sure how the math works out for that given the rope is flexible and curved over a relatively large area. I’d imagine it’s negligible though given the surface area of the pulley.
What is so hard to understand? The first and second anchor points support the load equally. The second anchor point also supports the force you are applying to lift the load. That’s why the first anchor point is 50 and the second is 100.
“A pulley system is a simple machine that helps you lift heavy objects more easily. It consists of a wheel with a rope or cable running over it. Here’s how it works in simple terms:
1. **Single Pulley**: Imagine you have a pulley attached to a ceiling. If you attach a rope to the object you want to lift and pull on one side of the rope, the pulley helps you lift the object with less effort. This works because the pulley changes the direction of your force—if you pull down, the object goes up.
2. **Mechanical Advantage**: If you use more than one pulley, you can make lifting even easier. A system with multiple pulleys, called a “block and tackle,” reduces the amount of force needed to lift an object by spreading the load across several ropes. This means you can lift heavier things with less effort, although you might need to pull the rope over a longer distance.
In short, pulleys help by changing the direction of your effort or distributing the weight, making tasks like lifting or moving heavy objects much easier.”
[https://www.explainthatstuff.com/pulleys.html#how-work](https://www.explainthatstuff.com/pulleys.html#how-work)
100kg on the B side it is more than 50kg because there is a monkey pulling rope down with the force of 50 newtons ADDED to the 50 kilo it is already hoisted on it.
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Shared work load
For every pulley you add 50k of force needed to pick up 100k. You would need 150k of force to pick up 100k in the picture shown – just add up all the downward forces.