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jt512
Aug 10, 2010, 8:35 PM
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redlude97 wrote: jt512 wrote: redlude97 wrote: Yes you are right I shouldn't have been so general in my statement. But the multiplication of force in an ADT on one of the anchor points is a pulley effect. If you say so. Like I said, no physics reference that I have ever seen defines or refers to such a "pulley effect." Jay Fine, would you be happier if I said a pulley by definition has a force multiplying effect?
No, because it doesn't, unless you include in your definition the trivial case of a single fixed pulley having a force multiplier of 1; that is, output force = 1 x (input force).
Jay
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redlude97
Aug 10, 2010, 8:54 PM
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jt512 wrote: redlude97 wrote: jt512 wrote: redlude97 wrote: Yes you are right I shouldn't have been so general in my statement. But the multiplication of force in an ADT on one of the anchor points is a pulley effect. If you say so. Like I said, no physics reference that I have ever seen defines or refers to such a "pulley effect." Jay Fine, would you be happier if I said a pulley by definition has a force multiplying effect? No, because it doesn't, unless you include in your definition the trivial case of a single fixed pulley having a force multiplier of 1; that is, output force = 1 x (input force). Jay
In reply to: Fundamental Concept 2: Pulleys are force magnifiers! When the two weighted legs of the rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled! Said another way, whenever a pulley is used, it automatically transmits an increased force to its point of attachment, by a ratio of 2:1. (We’ll find out why a bit later.)
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kjaking
Aug 10, 2010, 9:15 PM
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Take a look at the attachments. The original post is exactly this case, where the force on the upper bolt is doubled with this configuration. Its probably bomber enough, but it shouldn't be the first setup you look for.
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Rudmin
Aug 10, 2010, 9:24 PM
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jt512
Aug 10, 2010, 9:31 PM
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redlude97 wrote: jt512 wrote: redlude97 wrote: jt512 wrote: redlude97 wrote: Yes you are right I shouldn't have been so general in my statement. But the multiplication of force in an ADT on one of the anchor points is a pulley effect. If you say so. Like I said, no physics reference that I have ever seen defines or refers to such a "pulley effect." Jay Fine, would you be happier if I said a pulley by definition has a force multiplying effect? No, because it doesn't, unless you include in your definition the trivial case of a single fixed pulley having a force multiplier of 1; that is, output force = 1 x (input force). Jay Please take a look at this PDF and tell me if it is wrong then. I don't have any of my physics books on hand but the concept should be the same. In reply to: Fundamental Concept 2: Pulleys are force magnifiers! When the two weighted legs of the rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled! Said another way, whenever a pulley is used, it automatically transmits an increased force to its point of attachment, by a ratio of 2:1. (We’ll find out why a bit later.)
The author is confused. In the very next sentence he states that this [single fixed pulley] is a "good thing" because the pulley "doubles the force, helping us to move the object." Then, in so-called "Fundamental Concept 3" he implies that this is 2:1 mechanical advantage, which it isn't. Note that the article is written by somebody who puts "Esq" after his name. That means he's a lawyer, and he wants us to know it (perhaps he's warning the reader that what follows is not likely to be good physics).
A single fixed pulley doesn't really "do" anything, except change the direction of force needed to hold a weight. It doesn't help you move an object attached to it. Moving the object requires the same amount of force as if you had attached the rope to it directly.
Jay
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Rudmin
Aug 10, 2010, 9:43 PM
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jt512 wrote: redlude97 wrote: jt512 wrote: redlude97 wrote: jt512 wrote: redlude97 wrote: Yes you are right I shouldn't have been so general in my statement. But the multiplication of force in an ADT on one of the anchor points is a pulley effect. If you say so. Like I said, no physics reference that I have ever seen defines or refers to such a "pulley effect." Jay Fine, would you be happier if I said a pulley by definition has a force multiplying effect? No, because it doesn't, unless you include in your definition the trivial case of a single fixed pulley having a force multiplier of 1; that is, output force = 1 x (input force). Jay Please take a look at this PDF and tell me if it is wrong then. I don't have any of my physics books on hand but the concept should be the same. In reply to: Fundamental Concept 2: Pulleys are force magnifiers! When the two weighted legs of the rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled! Said another way, whenever a pulley is used, it automatically transmits an increased force to its point of attachment, by a ratio of 2:1. (We’ll find out why a bit later.) The author is confused. In the very next sentence he states that this [single fixed pulley] is a "good thing" because the pulley "doubles the force, helping us to move the object." Then, in so-called "Fundamental Concept 3" he implies that this is 2:1 mechanical advantage, which it isn't. Note that the article is written by somebody who puts "Esq" after his name. That means he's a lawyer, and he wants us to know it (perhaps he's warning the reader that what follows is not likely to be good physics). A single fixed pulley doesn't really "do" anything, except change the direction of force needed to hold a weight. It doesn't help you move an object attached to it. Moving the object requires the same amount of force as if you had attached the rope to it directly. Jay
Sounds like you are the one who is confused. Everything the author said is correct. He very clearly states that the forces at the pulleys point of attachment are doubled. This is true. He also states that attaching a pulley to an object helps to life that object. Also true. Note that the pulley is attached to the object, and not fixed to the frame of reference. You just can't read or can't understand. The "single fixed pulley" is your creation not his. He has perfectly described a 2:1 haul.
You are the one who can't think.
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jt512
Aug 10, 2010, 9:46 PM
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OK, you're right, he does say that if we attach the pulley to the object it helps us move it. I misread that.
Jay
(This post was edited by jt512 on Aug 10, 2010, 9:48 PM)
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Rudmin
Aug 10, 2010, 9:47 PM
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jt512 wrote: A single fixed pulley doesn't really "do" anything, except change the direction of force needed to hold a weight. It doesn't help you move an object attached to it. Moving the object requires the same amount of force as if you had attached the rope to it directly. Jay
A single fixed pull does "do" something more than chance the direction of a rope. It increases the force on whatever is "fixing" that pulley. In climbing cases that is often a bolt or piece of protection. The pulley in this case is increasing the load on the protection. "Pulley effect" is not a misnomer.
You are stuck thinking in terms of the climber being the load. The bolt is the load, the climber is the applied force.
What exactly do you engineer?
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jt512
Aug 10, 2010, 9:51 PM
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Rudmin wrote: jt512 wrote: A single fixed pulley doesn't really "do" anything, except change the direction of force needed to hold a weight. It doesn't help you move an object attached to it. Moving the object requires the same amount of force as if you had attached the rope to it directly. Jay A single fixed pull does "do" something more than chance the direction of a rope. It increases the force on whatever is "fixing" that pulley.
I don't really see it as the pulley increasing the force. The way I see it is that if you have to apply a force equal to the object's weight in order to keep it stationary. That's you doing something, not the pulley. Maybe it's just semantics, but there is probably a good reason why physicists don't think of this as "pulley effect." It's just a fact from elementary statics.
Jay
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spikeddem
Aug 10, 2010, 9:57 PM
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jt512 wrote: Rudmin wrote: Like I said, climbers use the term "pulley effect" all the time. The term seems unnecessary if you understand basic statics, and it seems to inevitably lead to confusion with mechanical advantage. Jay
Calling the left carabiner "A" and the right one "B":
It's true that the rope will not pull on A with anything but a re-directed force of the same magnitude (no mechanical advantage here). That said, since "A" must resist that force (equally, but opposite in magnitude?), the total force that "B" feels would be the force of the rope plus the (equivalent) force that "A" is resisting the rope with. That is to say, "B" feels a downward pull of 2x rope force (mechanical advantage here, yes?).
It certainly seems to me that this is the general idea that climbers generally as "the pulley effect." This becomes more obvious if you imagine that the side of the sling without an arrow was not present.
Why does it even matter whether or not "the pulley effect" is defined outside of the context in which we're speaking? I mean, it's basically accepted climber jargon.
Every single person knew what was meant by the first mention of "the pulley effect" so it seems like a waste of time to even argue about the term's legitimacy.
Edit: Too late. Oh well.
(This post was edited by spikeddem on Aug 10, 2010, 9:59 PM)
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dugl33
Aug 10, 2010, 10:02 PM
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Rudmin wrote: Please take a look at this PDF and tell me if it is wrong then. I don't have any of my physics books on hand but the concept should be the same. In reply to: Fundamental Concept 2: Pulleys are force magnifiers! When the two weighted legs of the
rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled! Said another way, whenever a pulley is used, it automatically transmits an increased force to its point of attachment, by a ratio of 2:1. (We’ll find out why a bit later.)
This is poorly written. If I hang 100 lbs on one strand and 100 lbs on the other, I've started with 100 + 100 = 200 lbs. So 200 total lbs of crap equaling 200 lbs on the anchor isn't really a force magnifier in my book. I could just as easily omit the pulley, still 200 lbs of crap loading the anchor.
The author clarifies frame of reference a bit later on with traveling vs. fixed pulleys.
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jt512
Aug 10, 2010, 10:11 PM
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dugl33 wrote: Rudmin wrote: Please take a look at this PDF and tell me if it is wrong then. I don't have any of my physics books on hand but the concept should be the same. In reply to: Fundamental Concept 2: Pulleys are force magnifiers! When the two weighted legs of the
rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled! Said another way, whenever a pulley is used, it automatically transmits an increased force to its point of attachment, by a ratio of 2:1. (We’ll find out why a bit later.) This is poorly written. If I hang 100 lbs on one strand and 100 lbs on the other, I've started with 100 + 100 = 200 lbs. So 200 total lbs of crap equaling 200 lbs on the anchor isn't really a force magnifier in my book. I could just as easily omit the pulley, still 200 lbs of crap loading the anchor.
Thanks. That is what I have been trying to communicate, albeit poorly.
Jay
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spikeddem
Aug 10, 2010, 10:16 PM
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jt512 wrote: dugl33 wrote: Rudmin wrote: Please take a look at this PDF and tell me if it is wrong then. I don't have any of my physics books on hand but the concept should be the same. In reply to: Fundamental Concept 2: Pulleys are force magnifiers! When the two weighted legs of the
rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled! Said another way, whenever a pulley is used, it automatically transmits an increased force to its point of attachment, by a ratio of 2:1. (We’ll find out why a bit later.) This is poorly written. If I hang 100 lbs on one strand and 100 lbs on the other, I've started with 100 + 100 = 200 lbs. So 200 total lbs of crap equaling 200 lbs on the anchor isn't really a force magnifier in my book. I could just as easily omit the pulley, still 200 lbs of crap loading the anchor. Thanks. That is what I have been trying to communicate, albeit poorly. Jay
All I can say to both of the two responses is that whoever first said--and whoever agreed with them--that the pulley effect was occurring here in the ADT, was not wrong. At least not on that part.
It's true, there's no magnification of force (more like a summation of force?), but there never has been in our climbing term "the pulley effect."
(This post was edited by spikeddem on Aug 10, 2010, 10:17 PM)
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jt512
Aug 10, 2010, 10:24 PM
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spikeddem wrote: jt512 wrote: Rudmin wrote: [image]http://img42.imageshack.us/img42/70/pulley.jpg[/image] Like I said, climbers use the term "pulley effect" all the time. The term seems unnecessary if you understand basic statics, and it seems to inevitably lead to confusion with mechanical advantage. Jay Calling the left carabiner "A" and the right one "B": It's true that the rope will not pull on A with anything but a re-directed force of the same magnitude ( no mechanical advantage here). That said, since "A" must resist that force (equally, but opposite in magnitude?), the total force that "B" feels would be the force of the rope plus the (equivalent) force that "A" is resisting the rope with. That is to say, "B" feels a downward pull of 2x rope force ( mechanical advantage here, yes?).
Sorry, but I don't follow that at all.
In reply to: Why does it even matter whether or not "the pulley effect" is defined outside of the context in which we're speaking?
Because it implies that a single fixed pulley is a force multiplier, which it isn't (see dugl33's post), and it inevitably leads to confusion with "mechanical advantage" (inevitable in the sense that in practically every thread mentioning "pulley effect" there is someone claiming that the so-called pulley effect is the same as mechanical advantage).
Jay
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whipper
Aug 10, 2010, 10:28 PM
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Thank G-d I worked all day, and was able to stay out of this. I am glad this discussion is going on though, to many climbers worry about this "pulley effect" with out even understanding basic mech advantage systems. Who is teaching what in school these days?
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redlude97
Aug 10, 2010, 11:18 PM
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dugl33 wrote: Rudmin wrote: Please take a look at this PDF and tell me if it is wrong then. I don't have any of my physics books on hand but the concept should be the same. In reply to: Fundamental Concept 2: Pulleys are force magnifiers! When the two weighted legs of the
rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled! Said another way, whenever a pulley is used, it automatically transmits an increased force to its point of attachment, by a ratio of 2:1. (We’ll find out why a bit later.) This is poorly written. If I hang 100 lbs on one strand and 100 lbs on the other, I've started with 100 + 100 = 200 lbs. So 200 total lbs of crap equaling 200 lbs on the anchor isn't really a force magnifier in my book. I could just as easily omit the pulley, still 200 lbs of crap loading the anchor. The author clarifies frame of reference a bit later on with traveling vs. fixed pulleys.
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jt512
Aug 10, 2010, 11:46 PM
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redlude97 wrote: dugl33 wrote: Rudmin wrote: Please take a look at this PDF and tell me if it is wrong then. I don't have any of my physics books on hand but the concept should be the same. In reply to: Fundamental Concept 2: Pulleys are force magnifiers! When the two weighted legs of the
rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled! Said another way, whenever a pulley is used, it automatically transmits an increased force to its point of attachment, by a ratio of 2:1. (We’ll find out why a bit later.) This is poorly written. If I hang 100 lbs on one strand and 100 lbs on the other, I've started with 100 + 100 = 200 lbs. So 200 total lbs of crap equaling 200 lbs on the anchor isn't really a force magnifier in my book. I could just as easily omit the pulley, still 200 lbs of crap loading the anchor. The author clarifies frame of reference a bit later on with traveling vs. fixed pulleys. That is because you are not attaching any weight to the pulley itself. In that case it is just a redirect as has already been explained previously. If you hang a 200lb weight from the a single pulley with the other end of the rope fixed, it only takes you 100lb of force to lift that 200lb item. That is magnification of force/mechanical advantage/whatever you want to call it.
The point is that the second bolded sentence does not support the first. If it did, we could say, for the same reason, that a bolt was a force magnifier. In fact, the exclamation point at the end of the second sentence is rather comical, since all the author is proving is that if you hang twice the weight off any point of attachment, the force on that point of attachment is doubled (!).
Jay
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redlude97
Aug 11, 2010, 12:12 AM
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jt512 wrote: redlude97 wrote: dugl33 wrote: Rudmin wrote: Please take a look at this PDF and tell me if it is wrong then. I don't have any of my physics books on hand but the concept should be the same. In reply to: Fundamental Concept 2: Pulleys are force magnifiers! When the two weighted legs of the
rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled! Said another way, whenever a pulley is used, it automatically transmits an increased force to its point of attachment, by a ratio of 2:1. (We’ll find out why a bit later.) This is poorly written. If I hang 100 lbs on one strand and 100 lbs on the other, I've started with 100 + 100 = 200 lbs. So 200 total lbs of crap equaling 200 lbs on the anchor isn't really a force magnifier in my book. I could just as easily omit the pulley, still 200 lbs of crap loading the anchor. The author clarifies frame of reference a bit later on with traveling vs. fixed pulleys. That is because you are not attaching any weight to the pulley itself. In that case it is just a redirect as has already been explained previously. If you hang a 200lb weight from the a single pulley with the other end of the rope fixed, it only takes you 100lb of force to lift that 200lb item. That is magnification of force/mechanical advantage/whatever you want to call it. The point is that the second bolded sentence does not support the first. If it did, we could say, for the same reason, that a bolt was a force magnifier. In fact, the exclamation point at the end of the second sentence is rather comical, since all the author is proving is that if you hang twice the weight off any point of attachment, the force on that point of attachment is doubled (!). Jay
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jt512
Aug 11, 2010, 12:48 AM
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redlude97 wrote: jt512 wrote: redlude97 wrote: dugl33 wrote: Rudmin wrote: Please take a look at this PDF and tell me if it is wrong then. I don't have any of my physics books on hand but the concept should be the same. In reply to: Fundamental Concept 2: Pulleys are force magnifiers! When the two weighted legs of the
rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled! Said another way, whenever a pulley is used, it automatically transmits an increased force to its point of attachment, by a ratio of 2:1. (We’ll find out why a bit later.) This is poorly written. If I hang 100 lbs on one strand and 100 lbs on the other, I've started with 100 + 100 = 200 lbs. So 200 total lbs of crap equaling 200 lbs on the anchor isn't really a force magnifier in my book. I could just as easily omit the pulley, still 200 lbs of crap loading the anchor. The author clarifies frame of reference a bit later on with traveling vs. fixed pulleys. That is because you are not attaching any weight to the pulley itself. In that case it is just a redirect as has already been explained previously. If you hang a 200lb weight from the a single pulley with the other end of the rope fixed, it only takes you 100lb of force to lift that 200lb item. That is magnification of force/mechanical advantage/whatever you want to call it. The point is that the second bolded sentence does not support the first. If it did, we could say, for the same reason, that a bolt was a force magnifier. In fact, the exclamation point at the end of the second sentence is rather comical, since all the author is proving is that if you hang twice the weight off any point of attachment, the force on that point of attachment is doubled (!). Jay How? Without the pulley, your 100lb force you exert can only lift 100lbs. With the pulley you can lift 200lbs. You have now multiplied your effective force by 2.
The author argues: "When the two weighted legs of the rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled." Therefore, "[p]ulleys are force magnifiers."
The first sentence is true, but the second sentence does not follow from it. If it did then it would also be valid to argue the following: When two weighted strands of rope are fixed to a bolt, the force is doubled. Therefore bolts are force magnifiers. As above, the first sentence is true, but the second sentence does not follow from it.
In neither case is the pulley or the bolt acting as a force multiplier. In both cases the force on the bolt or pulley are 2w by the simple fact that you've got two strands of rope hanging from the bolt or pulley, each with a force of w on it.
Jay
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redlude97
Aug 11, 2010, 1:15 AM
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jt512 wrote: redlude97 wrote: jt512 wrote: redlude97 wrote: dugl33 wrote: Rudmin wrote: Please take a look at this PDF and tell me if it is wrong then. I don't have any of my physics books on hand but the concept should be the same. In reply to: Fundamental Concept 2: Pulleys are force magnifiers! When the two weighted legs of the
rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled! Said another way, whenever a pulley is used, it automatically transmits an increased force to its point of attachment, by a ratio of 2:1. (We’ll find out why a bit later.) This is poorly written. If I hang 100 lbs on one strand and 100 lbs on the other, I've started with 100 + 100 = 200 lbs. So 200 total lbs of crap equaling 200 lbs on the anchor isn't really a force magnifier in my book. I could just as easily omit the pulley, still 200 lbs of crap loading the anchor. The author clarifies frame of reference a bit later on with traveling vs. fixed pulleys. That is because you are not attaching any weight to the pulley itself. In that case it is just a redirect as has already been explained previously. If you hang a 200lb weight from the a single pulley with the other end of the rope fixed, it only takes you 100lb of force to lift that 200lb item. That is magnification of force/mechanical advantage/whatever you want to call it. The point is that the second bolded sentence does not support the first. If it did, we could say, for the same reason, that a bolt was a force magnifier. In fact, the exclamation point at the end of the second sentence is rather comical, since all the author is proving is that if you hang twice the weight off any point of attachment, the force on that point of attachment is doubled (!). Jay How? Without the pulley, your 100lb force you exert can only lift 100lbs. With the pulley you can lift 200lbs. You have now multiplied your effective force by 2. The author argues: "When the two weighted legs of the rope passing through a pulley, are maintained in a parallel configuration, the forces brought to bear on the pulley’s point of attachment are doubled." Therefore, "[p]ulleys are force magnifiers." The first sentence is true, but the second sentence does not follow from it. If it did then it would also be valid to argue the following: When two weighted strands of rope are fixed to a bolt, the force is doubled. Therefore bolts are force magnifiers. As above, the first sentence is true, but the second sentence does not follow from it. In neither case is the pulley or the bolt acting as a force multiplier. In both cases the force on the bolt or pulley are 2 w by the simple fact that you've got two strands of rope hanging from the bolt or pulley, each with a force of w on it. Jay
If both ropes are fixed to the bolt, only 1 rope has a tension equal to W. The other strand would be under no tension, and the bolt would have a force equal to W. The fact that the bolt acts as a pulley(both strands connected running over the pulley) is what is necessary to cause tension in both lines doubling the force on the bolt.
In an ADT a similar situation is present. If you girth/clove both anchor bolts to fix the 3 strands, you don't get the force multiplication that you do in the situation where you run the sling directly through the 2 anchors so they act as pulleys. By not fixing the slings to the points, and allowing then to move freely(with friction) over the bolts/biners they act as pulleys, and the overall force on each bolt is higher. I call that multiplication of force by the pulley(s). What would you call that and would you agree that is what is occurring at the bolts?
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jt512
Aug 11, 2010, 3:00 AM
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redlude97 wrote: If both ropes are fixed to the bolt, only 1 rope has a tension equal to W. The other strand would be under no tension, and the bolt would have a force equal to W.
No, in general the ropes have tensions equal to w1 and w2, and the force on the bolt is w1 + w2.
In reply to: The fact that the bolt acts as a pulley(both strands connected running over the pulley) is what is necessary to cause tension in both lines doubling the force on the bolt.
No. The fact that in the specific case you cite w2 = w1 is the reason that the force on the pulley is 2 × w1. Whether the attachment point is a bolt or a pulley, the force on the attachment point will always be w1 + w2. Furthermore, w2 need not equal w1, whether the attachment point is a bolt or a pulley.
Jay
(This post was edited by jt512 on Aug 11, 2010, 3:00 AM)
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milesenoell
Aug 11, 2010, 4:49 AM
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Toast_in_the_Machine wrote: jt512 wrote: Toast_in_the_Machine wrote: jt512 wrote: redlude97 wrote: Yes no mechanical advantage. I shouldn't have ever used that term. Force mutliplication, even on a fixed point at a redirect is by definition a pulley effect though. As far as I am aware nothing is "by definition" a pulley effect, because, as far as I am aware, "pulley effect" is not a real term in physics or engineering. I've never seen the term used by anyone other than climbers on the internet. Jay Jay - prepare to be amazed: http://pubs.acs.org/...bs/10.1021/ma061037s I'm not even remotely amazed. They are not using the term "pulley effect" to mean whatever it is that climbers think it means. The linked article wrote: The SR gels have a unique characteristic called the “pulley effect” that the cross-links made of α-cyclodextrin molecules in a figure-eight shape can slide along the polymer chain. Jay Quit shooting at everything that moves Dick Cheney. I was helping your point. "Pully effect" only shows up in translated chemical articles and climbing sites.
I don't know, we've got a "figure eight", and some "chains" to go with the "pulley effect". If cross links are anything like quick links I think we might be onto something here.
(This post was edited by milesenoell on Aug 11, 2010, 4:51 AM)
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Rudmin
Aug 11, 2010, 3:11 PM
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dugl33 wrote: This is poorly written. If I hang 100 lbs on one strand and 100 lbs on the other, I've started with 100 + 100 = 200 lbs. So 200 total lbs of crap equaling 200 lbs on the anchor isn't really a force magnifier in my book. I could just as easily omit the pulley, still 200 lbs of crap loading the anchor.
It is not poorly written.
The pulley is a force magnifier and very different than simply hanging loads directly. The important difference is that an ideal (frictionless) pulley will always have the exact same tension on either strand going to the pulley. This means that whatever load you are able to apply to one strand will also be taken by the other strand and the pulley doubles the force. If you don't anchor the other end you can't apply much force, but you will still double whatever you can apply.
In your example of two hanging 100 lb weights, there is a big difference between a pulley and direct attachment. With the pulley, we can look at just one strand and automatically know how much load the anchor is holding. We see one strand holding 100 lbs, and we know there is a pulley at the top, so therefore the anchor is holding 200 lbs no matter what is going on at the other end.
With the attached weights, if we look at only one strand, we have no idea what kind of load the anchor is taking because we don't know how the other strand is loaded. We only know that it must be something more than 100 lbs.
The important fact is that both strands MUST have the same tension when they are joined by a pulley.
jt512 wrote: No. The fact that in the specific case you cite w2 = w1 is the reason that the force on the pulley is 2 × w1. Whether the attachment point is a bolt or a pulley, the force on the attachment point will always be w1 + w2. Furthermore, w2 need not equal w1, whether the attachment point is a bolt or a pulley.
This is only true for a bolt. It is absolutely incorrect for a pulley w1 MUST equal w2. You cannot apply different magnitudes of load to either strand. If you try to put different loads on either strand of a pulley, both side will accelerate at a rate proportional to the difference in weight that allows the loads to be equal, and the pulley will still experience the same tension in both strands.
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majid_sabet
Aug 11, 2010, 3:17 PM
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redlude97 wrote: styndall wrote: redlude97 wrote: styndall wrote: redlude97 wrote: styndall wrote: Rudmin wrote: whipper wrote: kjaking wrote: I am surprised that nobody has thought to comment on force multiplication due to pulley effects. The belayer himself is putting more than his body weight on the lower bolt. But I guess he would be pulled upward in a leader fall anyways, so w/e. If the climber had clipped through what the leader is using to anchor himself, there would be big problems in a fall. FAIL There is no pulley effect, a pulley requires a mechanical advantage, there is none. a lot of n00bs on here seem to misunderstand pulley systems. always ask yourself if I pull 1 foot of rope here, how high does the load move....if it is 1 foot, then there is no multiplication of forces, it is a 1 to 1 redirect. Any way it doesnt even matter in this case, as the load is split (albeit poorly) due ot the clove hitch pulling down on the lower bolt. I wouldnt set it up like this, but I wouldn't give a fuck if my partner did...it is not going to fail. Troll or stupid or both. That top bolt is acting as a pulley, same as in American Triangle. Nothing in the ADT acts as a pulley. I would consider multiplication of force a pulley effect Then you should find a dictionary and look up the word pulley. –noun, plural -leys. 1.a wheel, with a grooved rim for carrying a line, that turns in a frame or block and serves to change the direction of or to transmit force, as when one end of the line is pulled to raise a weight at the other end: one of the simple machines. 2.a combination of such wheels in a block, or of such wheels or blocks in a tackle, to increase the force applied. 3.a wheel driven by or driving a belt or the like, used to deliver force to a machine, another belt, etc., at a certain speed and torque. http://dictionary.reference.com/browse/pulleyDid you not notice the requirement of the presence of objects fitting meaning 1 in meaning 2? Because you're clearly literate. Your definition means that most simple machines are pulleys. You can define pulley that way if you want, but it's not a usual or useful definition. So are you saying carabiners used in a haul system are not acting as pulleys? Have you ever set up a slackline? If you don't think the carabiners are acting as pulleys in that case then yes, we won't agree on this topic, but almost everyone else will consider it a pulley system. The wheel is there to reduce friction yes, but no totally eliminate it. The carabiner works in the same way just with more friction. To say one is a pulley and the other is not when rigged in the same way IMO doesn't make any sense. They still give you the same mechanical advantage(minus the loss to friction).
The efficiency of pulley is about 0.87-0.95 so you are loosing very little when working with mechanical advantages.
for carabiner is about 0.4-0.5 so almost 50% of the pulling energy is getting wasted due to friction. This is critical when constructing complex mechanical advantages.
edit to add: the diameter of wheel and type of bearing on the pulley is the most important part of the pulley. the larger wheel gives you the best MA vs smaller size wheel.
(This post was edited by majid_sabet on Aug 11, 2010, 3:20 PM)
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