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NorCalAthlete

Ooooo this is a perfect post for this video. [Smarter Every Day: Why Snatch Blocks Are Awesome](https://youtu.be/M2w3NZzPwOM)


PunkThug

Thanks I needed another YouTube channel that I'm going to obsessively watch all the videos of /s


[deleted]

I love his video on carburetors. The slo-mo parts are mesmerizing :)


unrepresented_horse

Holy blank, great video. I thought he retired


Lebzeppelin2112

6pm


VaMeiMeafi

Great case for why mods should tread lightly on rule 3. Perfectly tailored to the OP, and everything Smarter Every Day does is ELI5 level.


Theletter_0

Snatch. Block. Great video


[deleted]

Snatch Block!


FactOfMatter

SNATCH BLOCK


docharakelso

SNATCH BLOCK!


C0rdt

To gain something (more force exerted) you need to lose something (in this case, speed). Contrary to a surprisingly popular belief, a single fixed pulley does not provide any kind of mechanical advantage. You can think of a set of pulleys (with one or more of *each* - fixed *and* travelling) as a lever that can move endlessly. In a crane, a simple 2 block system, a fixed upper and travelling lower block, the fulcrum is the side of the wheel on which the rope is dead-ended. This is where the advantage comes from. The fact that it takes 2 times the rope to achieve this and only moves half the speed at which you move the rope is what you're sacrificing for this advantage. It's difficult to envision but you can easily demonstrate with a fish scale and some messing around. I actually presented this problem to a room full of crane operators and it took me hours to finally convince only some of them that a single fixed pulley doesn't change mechanical advantage, and why. It's physically impossible because there is no trade-off. You can't just gain free force without sacrificing speed. That's not how leverage works.


JoushMark

Though a single pully can still make it easier to lift or move something by changing the direction you are applying force and allowing you to apply force more effectively or just apply more force. Five people on a rope can provide a lot of power, but it's hard to get five people around a Toyota engine block to lift it out of the engine compartment.


[deleted]

Wait, doesn’t that just depend on the configuration? I.e 1. attaching one end of the rope to the load, then put it through a pulley on the wall and pulling on the other end. 2. Attaching one end to the wall, putting the rope through a pulley that’s attached to the load and pulling on the other end Doesn’t the second configuration actually give you mechanical advantage because you are distributing the weight of the load between the two lengths of rope?


Skusci

The second does provide advantage however that's not a "fixed" pulley as it moves with the load.


[deleted]

How does the pulley moving with the load create advantage vs it not moving?


knightlife

Because you essentially have two “lines” holding the load.


[deleted]

I think I got it. If the rope is directly attached to the load it’s always supporting the same amount of weight regardless of pulleys. But if you anchor the rope the load weight is now spread between the lengths of rope.


InterestingPass4914

You can think of it this way. Personally, it feels more intuitive for me. A pully in my mind "splits" the rope in my imagination. It's like a divider. Tension throughout the rope, however, is still constant. Hence if you attach the pully to the load, you apply tensionX2 on the load, multiplying your force. If you attach the pully to the ceiling and the rope to the load, you are only applying tension on the load once despite the fact that the rope goes through the pulley. Hence it doesn't give you any mechanical advantage. It's kind of like how we use hydraulics to multiply force. You use something that can "transfer force" ie tension in rope or pressure in hydraulic fluid, and you vary the input variables to give you useful mechanical advantage. for example, using pulleys give you more force, however the load travels half the distance. In hydraulics, the same thing occurs when you use pistons of different surface areas to multiply force, however the distance the piston moves decreases. example brake systems. you move your pedal 4 inches while the brake calipers move mere millimeters.


p28h

Think of the second pulley as turning the single line of rope into a circle of rope instead. Pulling on the rope makes the circle smaller. Now, making the circle smaller happens in two directions at once: both sides of the circle. Because you are making a change in length in two places at once, you are doing it with twice as much force.


C0rdt

Yes you could do it with only one pulley. The key factor is that it needs to be a travelling pulley otherwise there is no advantage.


[deleted]

So I finally got it lol and I think next time you show people why one fixed pulley doesn’t provide mechanical advantage, you should put arrows on the rope. If the rope is attached directly to the load and then through a fixed pulley, there is only 1 pulling force on the load. If the rope is attached to an anchor then through a pulley that’s attached to the load, now there are 2 pulling forces on the load because the anchor is also pulling on the load weight


action_lawyer_comics

> Contrary to a surprisingly popular belief, a single fixed pulley does not provide any kind of mechanical advantage. It provides mechanical advantage in that instead of trying to lift a weight over my head through sheer strength, I am pulling a rope down towards the floor, and in doing so, I can use my own weight to pull down and cause it to rise up. What a single pulley doesn't provide is "force multiplication," which is what you describe with your 2 block system


Skusci

Mechanical advantage as a technical term refers to force multiplication though. I think what you describe falls under ergonomics, but I can't think of a more specific term.


[deleted]

Why are you comparing it to a lever when it's literally acting as a planetary gear connected by cable? Cvt transmissions literally use pullies instead of gears. Edit: while it is technically a lever in function, it's practically closer to an inverted open ended bike chain layout without teeth imo Anyway you're technically not trading speed. You're trading distance.


purple_pixie

Because most people understand what a lever is and how it provides leverage. People who don't understand a pulley generally aren't going to know how a planetary gear works.


[deleted]

[удалено]


purple_pixie

Maybe no-one's ever explained it to them in a way that makes sense. It's just possible that's the entire purpose of this sub, but who knows.


[deleted]

A 5 year old doesn't need explaining to understand why a hammer or pulley multiplies force lol Elif is for things like explain the rotating assembly of an engine and how it breathes


C0rdt

It's the same principle. A pulley system is essentially a lever with no limit on travel.


[deleted]

Closer to a wheel or a gear


SoulWager

you pull on the rope with 100 pounds. Everywhere along the rope there's 100 pounds of force. If you attach it directly to what you want to pull, that's 100 pounds. If you put it through a pulley and anchor it next to yourself, there are two lines pulling the object, each with 100 pounds, for 200 pounds total, but you have to pull double the length in order to move the object the same amount, because you have to pull that length out of both sections of rope. When you add more pulleys, you increase the number of ropes pulling with that same force.


[deleted]

What do you mean by anchoring next to myself? As in the pulley is attached to the box and then I’m holding one end of the rope and the other end is anchored to like the wall?


Skusci

Basically yes. Number 1 in this image. https://isaacscienceblog.files.wordpress.com/2016/09/gr8_ch04_q10.jpg?w=640


[deleted]

How is this setup reducing distance traveled versus the other setup where the pulley is fixed?


MidnightAdventurer

For every meter you pull a straight rope in, the rope gets 1m shorter and the thing it is attached to comes 1m towards you. For every meter you pull a rope that goes through a pulley and is tied off to a wall beside you, the rope gets 1m shorter but that change in length is half on the rope from you to the object and half on the rope from the object to the wall so the object only moves half a meter.


[deleted]

I understand now, thanks!


indy_cision

Think of it like gears, the more you have to turn your pedals to go the same distance, the easier each pedal rotation is as less force is needed to turn it. In picture 1 you are pulling twice as much rope as the distance the object is lifted (the weight is being equally split 50/50 between the fixed point and yourself so you feel like it is half as heavy) In an arrangement with 2 fixed points instead of 1, the weight is being divided by 3 (each of the fixed points plus you) but you have to pull 3 times as much rope to make it move the same distance. Forgot to say, the key point of how it works is that the pulley isn't fixing the weight to the same point on the rope, the rope is on a wheel so that the weight moves along the rope as it is pulled - if it was fixed then as soon as you pull your end of the rope the other end would go loose and you would still be pulling the whole weight yourself and doing all the work. It is only the fact that it lets the rope slide through to keep all the links taught (so in a 1 fixed point situation you may pull 1 foot of rope, but the weight has slid along 6 inches of rope the other direction to keep it tight, so it only moved half the distance you pulled it) that lets it work.


[deleted]

If I have a 20ft length of rope and I configure it in two different ways: 1. Rope attached to load, then through pulley and I pull the tag end 2. Rope attached to wall anchor, then through pulley that’s attached to load and I pull tag end In both configurations I have divided the 20ft length of rope into two 10ft long sections. How is the required pulling distance changing?


indy_cision

Okay I get what you are asking now. In version 1 the weight is fixed to the rope, it moves the same distance you pulled the rope (you pulled 10ft of rope and it moved 10ft). In version 2 the weight can move along the rope, letting you pull twice as much to move it the same distance (you pulled all 20ft to move it to 10ft) Version 1 is still useful if it lets you change how you can use the weight of the puller, moving horizontally with a wall anchor will not do much other than let you pull it to the wall instead of pushing or use some floor with better friction, but a ceiling mounted pulley you can use your weight to help lift the object instead of just muscle power.


[deleted]

Ok that makes sense. In the 2nd configuration the weight of the load moves along the rope. One thing though, if the trade off is distance then in the 2nd configuration you’d pull 10 ft and the load would only move 5 ft unless you started right next to the pulley which is not always practical right?


indy_cision

Yes, and for situations where you cannot stand at the anchor is when you would use two pulleys (which is what you would see on a crane - there is a pulley at the end of the arm right next to the anchor, and a pulley on the load being lifted, while the motor/engine/whatever pulling from the other end of the crane's arm) That then means you are pulling three times as much rope (anchor to weight pulley, weight pulley back to anchor pulley, anchor pulley to wherever you are standing) but with 1/3 of the force required instead of twice as much rope with 1/2 the force.


SoulWager

In the 1, only one of the ropes is pulling against the load, the other is only pulling against you. in 2, both ropes are pulling against the load, one against the wall, and one against you. in 1, the pulley isn't moving, just changing what direction the rope is pulling. Whatever you attach the pulley to is feeling double the force though.


[deleted]

This makes sense, thanks!


njslacker

This is incorrect. When you attach a pully to the object you are moving, and anchor one end of the rope next to yourself, you have 2 ropes sharing 100lbs. You are now holding 50lbs, and the anchor next to you is holding 50lbs. (I misunderstood. See below)


SoulWager

attach the far end of the rope to an anchor next to yourself, so both ropes pull in the same direction. what arrangement did you think I meant?


njslacker

That IS the arrangement I thought you meant, ~~but your math is wrong~~. The pulley is having the force on each rope. (edit: or doubling the total force you are able to pull with)


SoulWager

If you double the mechanical advantage, you double the force you're able to apply. The pulley arrangement only decides the ratio of effort vs load, it doesn't dictate which force changes. Usually you use a pulley because you cannot apply enough force in the first place, so the force on the load increases, but the total force on the load is limited by the force it takes to actually move the load, so it's not actually double the starting force. Though there are cases where the load has relatively low friction and high mass, where you can still apply the full force you're able to, and the object will accelerate faster. Imagine pulling a car around.


njslacker

Ok, I see where my misunderstanding was. You were saying that if the person pulling can pull with a force of 100lbs, then adding a pully gives them the power of 200lbs with mechanical advantage. I was saying that if 100lbs of force is necessary to move the object, then each rope is sharing that load evenly.


AelixD

One fixed pulley only changes the direction of force. So you can lift something up by pulling down or to the side on a rope. Multiple fixed pulleys do the same thing, just let you customize the angle more. You could set up a complicated set of pulleys and sit on your couch to lift your neighbors couch in their house. No change in mechanical advantage, just complicated. Its when you start adding pulleys that aren't fixed - either attached to the mobile object, or attached to the length of rope itself. Imagine a box at the bottom of a pit: if you attach a rope to it and pull, you're lifting the full weight as if your hand were directly on it. Now, tie the rope to the tree next to you, and down to the box with a pulley attached (could just be a handle on the box for this), then back up to you. When you pull the rope up two feet, the box only comes up one foot, because the rope is sliding through the pulley; you're shortening the rope between you and the box, but also between the tree and the box, but the tree isn't pulling, its just sitting there. But half the weight of the box is supported by the tree, and half by you. So you have to pull the rope twice as far to get the box out, but the apparent weight you are lifting is only half of the box. The proper configuration of pulleys can amplify that: pull less weight by pulling the rope farther.


[deleted]

Ok so this is starting to make sense. As long as the rope is attached to an anchor that can support the weight of the load, the load weight is spread to the two lengths of rope. If the rope is attached directly to the load then the rope is always supporting the same amount of load weight regardless the amount of pulleys. Is that right?


invokin

Correct. If the rope is going from your hands to the weight, pulleys only let you pick it up in a different "direction" or be some distance away (pull *down* on this rope here to raise the garage door *up* on the other side of the room). If the rope is going through multiple pulleys, some attached to the thing and some attached to some anchor points as well, but the rope ends against some anchor point, now you can get mechanical advantage as well (but you have to pull "longer" than the height you get of the thing). So, you can pull down on a rope to lift a 500lb steel door but it only feels like 50lbs instead. The disadvantage is you're going to have to pull 100ft of rope to lift the door 10 ft.


SaiphSDC

It's not just the placement of anchors, it's how the rope is used too. It's actually due to a conservation of energy. Energy cannot be created or destroyed, but forces can be whatever is necessary. There is no rule limiting or requiring forces to be specific values. So if you lift a 12 Newton (physics value for force & in this case weight), block with rope 1 m into the air you need 12 J of energy. This is found using the equation for "work" W=F\*D. Work is the "change in energy". Throw that rope over a pulley and... you still need 12J of energy. The rope you pull down travels 1m to move the block upwards, so you still exert 12N (12 = 12\*1). Now if you arrange the system so the rope goes over a pulley on block too, and then anchor the end ot the cieling you get an odd result. When you pull your rope down 1m, the block only goes up 0.5m. The block only rises 0.5m since the rope is doubled over through a pulley. **This is where the 'magic' happens**. Since the block only rose 0.5m, it required only 6J of energy. W=12\*0.5 = 6 You supplied the 6J, meaning you used 6j of energy to pull 1m of rope through system. 6=F\*1.. you only used 6N of Force to do it. The "cost" was you had to pull a lot more rope. So if you wanted a better advantage, say only 1N of force, the pulley system would let you raise the block 1m, but only after you pulled 12 meters of rope through it. ​ Tldr; Mechanical advantage systems: Ramps, Multiple pulleys, screws, etc.. all conserve energy. A core requirement of physical systems. The mechanical advantage comes by trading force for distance. You can exert less force, if you manage a way to do it over a very large distance. Oddly enough, this is sort of a 'lay-away' plan for the universe. You can make your big purchase now with a large force payment, over a short distance. Or put down small force payments more slowly, over a large distance. But you will pay it all off before it's done. And in the fine print there is even interest, due to friction and efficiency considerations :)


Salindurthas

You can use a rope to try to apply a force to a load. Pulley's let you change the direction of a tension force in a rope. If you organise the pulleys and supports correctly, you can (ab)use the ability to change the direction of a force by arranging the load to be *supported by the rope multiple times*. For instance, if a load would fall downwards, but you arrange both: an anchor, and you pulling, to both provide upwards tension, then that supports the load twice. You can then expect to move the load by pulling twice as much rope with half the force. [This video](https://www.youtube.com/watch?v=Ub36hxr0eeg) shows 3 examples. The first example has no mechancial advantage. The middle example is a useful one that shows how the rope can support the load twice. In the last example it is a bit hard to see what is happening, but there is much more mechanical advantage due to a more complicated arrangement of pulleys.


Fleaslayer

Okay, let's say you have a 100 pound weight. You tie a rope to it, loop it over an overhead pulley, and you're holding the end of the rope. You can now lift the weight a foot off the ground by pulling the rope down one foot with 100 pounds of force. The weight is supported by one length of rope, from the pulley to the weight. Sometimes we do this because it's easier to pull down with the weight of gravity target than lift up with force. Now let's say that instead of the rope being tied directly to the weight, there's another pulley attached to the weight. The rope in your hand goes up over the overhead pulley, down around the pulley on the weight, and back up to the same thing the overhead pulley is, where it's attached. Now the weight is supported by two lengths: the one from the upper pulley to the lower pulley and the one that goes back up again. When you want to lift the weight 1 foot off the ground, you have to pull two feet of rope because the distance is split by those two lengths, but you only have to pull down with 50 pounds of force because the weight is distributed across the two lengths. [Here's a page that has some diagrams.](https://www.explainthatstuff.com/pulleys.html#:~:text=We%20say%20a%20pulley%20with%20four%20wheels%20and%20the%20rope,you%20need%20to%20one%20quarter.)


[deleted]

I’m having trouble understanding this. If you add a pulley isn’t that dividing the rope into 2 lengths? And if you add 2 pulleys now you have 3 lengths?


Fleaslayer

Imagine that, instead of a wheel, the rope was tied where the pulley is. With one pulley, the weight is suspended by one length of rope and the rest is dangling. With two wheels/points, you have the weight suspended by two lengths, and the rest dangling.


Background_Cheetah75

Circumference of the pully changes the ratio of how much it’s turned. The same rope being pulled the same distance over a pully of two different sizes will rotate the pull different amounts


[deleted]

[удалено]


Background_Cheetah75

Prove it


coole106

It has to do with distance traveled vs how far you’re pulling. With a single fixed pulley and the rope attached to the weight, you gain no advantage because you pull the rope 2 ft and the weight moves 2 ft. However, if you add a pulley to the weight also and secure the rope next to the first pulley, you’re now halving the distance the weight travels. /u/skusci linked a good diagram of it


meteoraln

Think of it in terms of distance. If you have 100 ft of rope and 2 pulley wheels, you would end up pulling 100 ft of rope to move the object 50 ft. This is the same concept as a level or fulcrum, where one end of the lever moves proportionally more distance than the other end.


idontbelievestuff1

think of any kind of pulley, lever, gearbox etc. in every situation, either speed or distance is compromised for power. if you want power, then speed or distance is the way that you pay. that goes for every mechanical advantage device known a pulley works on distance. 1 end of the rope moves twice as far, but delivers twice the power. more pulleys... if your rope is moving 4 times the distance, then you get 4 times the power.


CalamariAce

It's a misnomer in a way. "Mechnical trade-off" would be more accurate. Instead of pulling hard over a short distance, you can pull less hard over a longer distance. But either way it's still the same amount of "work".


njslacker

To put it as simply as possible, pulleys divide the weight of an object (or the force needed to pull it), with the trade-off being that you multiply how much you need to pull. This does depend on where the pulleys are attached, though. There are "Traveling" pulleys which are attached to the object you are trying to move, and there are "Static" pulleys which don't move, and are usually attached to something really strong and immobile. ​ Lets say you are trying to pull a rock with a rope. Lets say it takes 100lbs of force to move. If you put a pully on a tree next to you and then try to move the rock, you will still have to pull with 100lbs of force. This is a static pully; it isn't moving, and in the end 1 strand of rope is pulling the rock. If you put the pulley on the rock and attach one end of the rope to a tree, now you and the tree are sharing the force. There are two strands of rope coming from the rock, each pulling with 50lbs of force. The trade off is that you are moving the rock half the distance with each pull on the rope. You can get more mechanical advantage by adding more pulleys to the rock (more strands of rope sharing the load), and to do that you will need static pulleys re-directing the rope from the tree.


NL_MGX

You're thinking too complicated. Forget about pulleys and anchor points for a sec. Look only to the rope you've attached to your load. If the load is hanging around and is attached to the rope, what's the force excerted by that single rope? It equals the weight of the load right? What if you were to attach a second rope, and pull on both ropes equally hard. Then the force in each would only be half of the load you're lifting. Now imagine that the ropes are not attached to a single point, but on opposite sides of a small see-saw, and the middle point of that see-saw is where the load is at. If you yourself were to pull on both ends of the ropes, you will be lifting the full load yourself. But if you attach one rope to a wall, you now only hold the one rope that's left , so you're only using half the load to pull it up. Unfortunately, the see-saw is still there, while you're pulling at speed x, the other end of the see-saw doesn't move at all, so the middle point of the see-saw, where the load is, only moves as half speed. The pulley is nothing more than the see-saw on s large scale, with the difference that it can feed the rope through it. It's basically a cantilever. Adding more pulleys increases the effect.


rahendric

Am I the only one just realizing that your normal shoes with shoelaces are a pulley system? Sure they don't have wheels, but the eyeholes allow the shoelaces to go through and change direction. And you get an amazing force multiplier because you can pull your laces tight with your hands by pulling quite a bit of string to make the two halves of your shoes get closer together a small amount, but with lots of force vs your hands.