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rgold
Aug 11, 2010, 3:19 PM
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Rudmin wrote: 
The discussion seems to be clouded by terms like "force multiplier" which depend on unstated (and perhaps, for some posters, unknown) assumptions. Without making those assumptions explicit, such terms are meaningless, which of course provides a wide latitude for arguments that can't be settled.
As far as the statics goes, for the mythical frictionless pulleys, anchor loads are determined by the fact that the tension in the sling is constant:
This is, as claimed earlier, a version of the so-called American Death Triangle, a configuration that, while not ideal, isn't nearly as bad as its name suggests in many situations.
This being one of them. This anchor is nothing to freak out over if the bolts are reasonable. But still, it isn't ideal; for the situation pictured, in-line clove hitches would have made much more sense.
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jt512
Aug 11, 2010, 4:40 PM
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Rudmin wrote: 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.
Therefore, you can apply different weights to the two strands of rope attached to a pulley, and the effect of doing so—the real "pulley effect"—is to lift the lighter weight. So a single fixed pulley does not multiply force. It differs from a fixed bolt in that a pulley allows the lighter weight to be lifted.
Jay
(This post was edited by jt512 on Aug 11, 2010, 4:41 PM)
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Toast_in_the_Machine
Aug 11, 2010, 4:49 PM
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milesenoell wrote: 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.
My god man - you are right!!!
This isn't actually a translation of a chem paper, it is the lost translation of a paper on the effects of weed on lead climbers!
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Rudmin
Aug 11, 2010, 4:49 PM
Post #104 of 140
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jt512 wrote: Rudmin wrote: 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. Therefore, you can apply different weights to the two strands of rope attached to a pulley, and the effect of doing so—the real "pulley effect"—is to lift the lighter weight. So a single fixed pulley does not multiply force. It differs from a fixed bolt in that a pulley allows the lighter weight to be lifted. Jay
No, you are not applying different weights. If the weight is falling, the pulley is not holding that weight any more than you would be benching 200 lbs by merely hang on to the barbell as it falls onto your chest. The pulley is holding the exact same force in each strand, it doesn't matter how fast they accelerate or move in either direction. If you just look at the pulley, each strand carries the same load.
I've explained it about as simply as I can manage and don't think you understand my point at all. And we have gotten so far from the OP that there is really no point to this anymore.
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jt512
Aug 11, 2010, 5:06 PM
Post #105 of 140
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Rudmin wrote: jt512 wrote: Rudmin wrote: 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. Therefore, you can apply different weights to the two strands of rope attached to a pulley, and the effect of doing so—the real "pulley effect"—is to lift the lighter weight. So a single fixed pulley does not multiply force. It differs from a fixed bolt in that a pulley allows the lighter weight to be lifted. Jay No, you are not applying different weights.
Sure I am. I'm hanging 200 lb (w1) on one side and 100 lb (w2) on the other.
In reply to: If the weight is falling, the pulley is not holding that weight any more than you would be benching 200 lbs by merely hang on to the barbell as it falls onto your chest.
Huh? The total downward force on the pulley is clearly 300 lb (w1 + w2, as I said up-thread).
In reply to: The pulley is holding the exact same force in each strand, it doesn't matter how fast they accelerate or move in either direction. If you just look at the pulley, each strand carries the same load.
Each strand may have the same tension, but that does not alter the fact that the total downward force on the pulley is the sum of the weights attached to each strand of the rope.
Jay
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redlude97
Aug 11, 2010, 5:08 PM
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jt512 wrote: 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
Is this 2 pulley system providing any mechanical advantage/force multiplication?
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snoboy
Aug 11, 2010, 5:11 PM
Post #107 of 140
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If you let T='Tension in sling', then the Anchor load will be a function of T, not L...
rgold wrote: As far as the statics goes, for the mythical frictionless pulleys, anchor loads are determined by the fact that the tension in the sling is constant: [img]http://usera.ImageCave.com/rgold/Technical/Anchor.png[/img] |
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redlude97
Aug 11, 2010, 5:12 PM
Post #108 of 140
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jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: 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. Therefore, you can apply different weights to the two strands of rope attached to a pulley, and the effect of doing so—the real "pulley effect"—is to lift the lighter weight. So a single fixed pulley does not multiply force. It differs from a fixed bolt in that a pulley allows the lighter weight to be lifted. Jay No, you are not applying different weights. Sure I am. I'm hanging 200 lb (w1) on one side and 100 lb (w2) on the other. In reply to: If the weight is falling, the pulley is not holding that weight any more than you would be benching 200 lbs by merely hang on to the barbell as it falls onto your chest. Huh? The total downward force on the pulley is clearly 300 lb (w1 + w2, as I said up-thread). In reply to: The pulley is holding the exact same force in each strand, it doesn't matter how fast they accelerate or move in either direction. If you just look at the pulley, each strand carries the same load. Each strand may have the same tension, but that does not alter the fact that the total downward force on the pulley is the sum of the weights attached to each strand of the rope. Jay
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Rudmin
Aug 11, 2010, 5:19 PM
Post #109 of 140
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jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: 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. Therefore, you can apply different weights to the two strands of rope attached to a pulley, and the effect of doing so—the real "pulley effect"—is to lift the lighter weight. So a single fixed pulley does not multiply force. It differs from a fixed bolt in that a pulley allows the lighter weight to be lifted. Jay No, you are not applying different weights. Sure I am. I'm hanging 200 lb (w1) on one side and 100 lb (w2) on the other. In reply to: If the weight is falling, the pulley is not holding that weight any more than you would be benching 200 lbs by merely hang on to the barbell as it falls onto your chest. In reply to: The pulley is holding the exact same force in each strand, it doesn't matter how fast they accelerate or move in either direction. If you just look at the pulley, each strand carries the same load. Each strand may have the same tension, but that does not alter the fact that the total downward force on the pulley is the sum of the weights attached to each strand of the rope. Jay
This is very wrong. Statics and dynamics Mr. engineer.
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jt512
Aug 11, 2010, 5:25 PM
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redlude97 wrote: Jay, Is this 2 pulley system providing any mechanical advantage/force multiplication?
MA = 2, but the fixed pulley, which is what we have been discussing, does not contribute to it.
Jay
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rgold
Aug 11, 2010, 5:25 PM
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snoboy wrote: If you let T='Tension in sling', then the Anchor load will be a function of T, not L... rgold wrote: As far as the statics goes, for the mythical frictionless pulleys, anchor loads are determined by the fact that the tension in the sling is constant:
Well sure...what is your point? Folks here seem to be interested in how the load imposed at the tie-in point is distributed to the anchors, which is what the diagram and formulas calculate.
The calculation in terms of T is visibly included in the post as Tcos(alpha/2). (Edited this to eliminate mathematical ambiguity.)
(This post was edited by rgold on Aug 11, 2010, 5:36 PM)
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jt512
Aug 11, 2010, 5:30 PM
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Rudmin wrote: jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: 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. Therefore, you can apply different weights to the two strands of rope attached to a pulley, and the effect of doing so—the real "pulley effect"—is to lift the lighter weight. So a single fixed pulley does not multiply force. It differs from a fixed bolt in that a pulley allows the lighter weight to be lifted. Jay No, you are not applying different weights. Sure I am. I'm hanging 200 lb (w1) on one side and 100 lb (w2) on the other. In reply to: If the weight is falling, the pulley is not holding that weight any more than you would be benching 200 lbs by merely hang on to the barbell as it falls onto your chest. In reply to: The pulley is holding the exact same force in each strand, it doesn't matter how fast they accelerate or move in either direction. If you just look at the pulley, each strand carries the same load. Each strand may have the same tension, but that does not alter the fact that the total downward force on the pulley is the sum of the weights attached to each strand of the rope. Jay This is very wrong. Statics and dynamics Mr. engineer.
How wrong? What's the total downward force on the pulley, "Mr. Engineer"?
Jay
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Rudmin
Aug 11, 2010, 5:37 PM
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jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: 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. Therefore, you can apply different weights to the two strands of rope attached to a pulley, and the effect of doing so—the real "pulley effect"—is to lift the lighter weight. So a single fixed pulley does not multiply force. It differs from a fixed bolt in that a pulley allows the lighter weight to be lifted. Jay No, you are not applying different weights. Sure I am. I'm hanging 200 lb (w1) on one side and 100 lb (w2) on the other. In reply to: If the weight is falling, the pulley is not holding that weight any more than you would be benching 200 lbs by merely hang on to the barbell as it falls onto your chest. In reply to: The pulley is holding the exact same force in each strand, it doesn't matter how fast they accelerate or move in either direction. If you just look at the pulley, each strand carries the same load. Each strand may have the same tension, but that does not alter the fact that the total downward force on the pulley is the sum of the weights attached to each strand of the rope. Jay This is very wrong. Statics and dynamics Mr. engineer. How wrong? What's the total downward force on the pulley, "Mr. Engineer"? Jay
I will let this highschool physics teacher explain it to you:
http://www.youtube.com/...&feature=related
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redlude97
Aug 11, 2010, 5:37 PM
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jt512 wrote: redlude97 wrote: Jay, Is this 2 pulley system providing any mechanical advantage/force multiplication? [image]http://static.howstuffworks.com/gif/bt3.gif[/image] MA = 2, but the fixed pulley, which is what we have been discussing, does not contribute to it. Jay
(This post was edited by redlude97 on Aug 11, 2010, 5:39 PM)
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Fixed pulley.JPG
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redlude97
Aug 11, 2010, 5:38 PM
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jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: 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. Therefore, you can apply different weights to the two strands of rope attached to a pulley, and the effect of doing so—the real "pulley effect"—is to lift the lighter weight. So a single fixed pulley does not multiply force. It differs from a fixed bolt in that a pulley allows the lighter weight to be lifted. Jay No, you are not applying different weights. Sure I am. I'm hanging 200 lb (w1) on one side and 100 lb (w2) on the other. In reply to: If the weight is falling, the pulley is not holding that weight any more than you would be benching 200 lbs by merely hang on to the barbell as it falls onto your chest. In reply to: The pulley is holding the exact same force in each strand, it doesn't matter how fast they accelerate or move in either direction. If you just look at the pulley, each strand carries the same load. Each strand may have the same tension, but that does not alter the fact that the total downward force on the pulley is the sum of the weights attached to each strand of the rope. Jay This is very wrong. Statics and dynamics Mr. engineer. How wrong? What's the total downward force on the pulley, "Mr. Engineer"? Jay
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Rudmin
Aug 11, 2010, 5:49 PM
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redlude97 wrote: jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: 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. Therefore, you can apply different weights to the two strands of rope attached to a pulley, and the effect of doing so—the real "pulley effect"—is to lift the lighter weight. So a single fixed pulley does not multiply force. It differs from a fixed bolt in that a pulley allows the lighter weight to be lifted. Jay No, you are not applying different weights. Sure I am. I'm hanging 200 lb (w1) on one side and 100 lb (w2) on the other. In reply to: If the weight is falling, the pulley is not holding that weight any more than you would be benching 200 lbs by merely hang on to the barbell as it falls onto your chest. In reply to: The pulley is holding the exact same force in each strand, it doesn't matter how fast they accelerate or move in either direction. If you just look at the pulley, each strand carries the same load. Each strand may have the same tension, but that does not alter the fact that the total downward force on the pulley is the sum of the weights attached to each strand of the rope. Jay This is very wrong. Statics and dynamics Mr. engineer. How wrong? What's the total downward force on the pulley, "Mr. Engineer"? Jay 200lbs, like i stated above.
Also wrong. A 200 lb mass and 100 lb mass hung on a theoretical pulley and allowed to accelerate indefinitely will exert 266.67 lbs on the pulley.
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jt512
Aug 11, 2010, 5:53 PM
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redlude97 wrote: jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: 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. Therefore, you can apply different weights to the two strands of rope attached to a pulley, and the effect of doing so—the real "pulley effect"—is to lift the lighter weight. So a single fixed pulley does not multiply force. It differs from a fixed bolt in that a pulley allows the lighter weight to be lifted. Jay No, you are not applying different weights. Sure I am. I'm hanging 200 lb (w1) on one side and 100 lb (w2) on the other. In reply to: If the weight is falling, the pulley is not holding that weight any more than you would be benching 200 lbs by merely hang on to the barbell as it falls onto your chest. In reply to: The pulley is holding the exact same force in each strand, it doesn't matter how fast they accelerate or move in either direction. If you just look at the pulley, each strand carries the same load. Each strand may have the same tension, but that does not alter the fact that the total downward force on the pulley is the sum of the weights attached to each strand of the rope. Jay This is very wrong. Statics and dynamics Mr. engineer. How wrong? What's the total downward force on the pulley, "Mr. Engineer"? Jay 200lbs, like i stated above.
We're both wrong, according to the video in Rudmin's link. Although that doesn't change the fact that a single fixed pulley is not a force multiplier.
Jay
(This post was edited by jt512 on Aug 11, 2010, 6:01 PM)
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redlude97
Aug 11, 2010, 6:32 PM
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Rudmin wrote: redlude97 wrote: jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: 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. Therefore, you can apply different weights to the two strands of rope attached to a pulley, and the effect of doing so—the real "pulley effect"—is to lift the lighter weight. So a single fixed pulley does not multiply force. It differs from a fixed bolt in that a pulley allows the lighter weight to be lifted. Jay No, you are not applying different weights. Sure I am. I'm hanging 200 lb (w1) on one side and 100 lb (w2) on the other. In reply to: If the weight is falling, the pulley is not holding that weight any more than you would be benching 200 lbs by merely hang on to the barbell as it falls onto your chest. In reply to: The pulley is holding the exact same force in each strand, it doesn't matter how fast they accelerate or move in either direction. If you just look at the pulley, each strand carries the same load. Each strand may have the same tension, but that does not alter the fact that the total downward force on the pulley is the sum of the weights attached to each strand of the rope. Jay This is very wrong. Statics and dynamics Mr. engineer. How wrong? What's the total downward force on the pulley, "Mr. Engineer"? Jay 200lbs, like i stated above. Also wrong. A 200 lb mass and 100 lb mass hung on a theoretical pulley and allowed to accelerate indefinitely will exert 266.67 lbs on the pulley. 
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redlude97
Aug 11, 2010, 6:35 PM
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jt512 wrote: redlude97 wrote: jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: jt512 wrote: Rudmin wrote: 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. Therefore, you can apply different weights to the two strands of rope attached to a pulley, and the effect of doing so—the real "pulley effect"—is to lift the lighter weight. So a single fixed pulley does not multiply force. It differs from a fixed bolt in that a pulley allows the lighter weight to be lifted. Jay No, you are not applying different weights. Sure I am. I'm hanging 200 lb (w1) on one side and 100 lb (w2) on the other. In reply to: If the weight is falling, the pulley is not holding that weight any more than you would be benching 200 lbs by merely hang on to the barbell as it falls onto your chest. In reply to: The pulley is holding the exact same force in each strand, it doesn't matter how fast they accelerate or move in either direction. If you just look at the pulley, each strand carries the same load. Each strand may have the same tension, but that does not alter the fact that the total downward force on the pulley is the sum of the weights attached to each strand of the rope. Jay This is very wrong. Statics and dynamics Mr. engineer. How wrong? What's the total downward force on the pulley, "Mr. Engineer"? Jay 200lbs, like i stated above. We're both wrong, according to the video in Rudmin's link. Although that doesn't change the fact that a single fixed pulley is not a force multiplier. Jay
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cracklover
Aug 11, 2010, 6:44 PM
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rgold wrote:
This is, as claimed earlier, a version of the so-called American Death Triangle, a configuration that, while not ideal, isn't nearly as bad as its name suggests in many situations. This being one of them. This anchor is nothing to freak out over if the bolts are reasonable. But still, it isn't ideal; for the situation pictured, in-line clove hitches would have made much more sense.
My physics is extremely rudimentary, so forgive me if I'm missing something obvious. But one implication of your equation above is that any time the two angles are equal, the force on the anchor is exactly L. Intuitively, this seems a little odd, since one could imagine a standard ADT anchor in which those two angles were equal (an equilateral triangle would do it) and in which we know that both bolts definitely see a force > L.
GO
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snoboy
Aug 11, 2010, 6:49 PM
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rgold wrote: The calculation in terms of T is visibly included in the post as Tcos(alpha/2). (Edited this to eliminate mathematical ambiguity.)
Oops, I see it now...
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Rudmin
Aug 11, 2010, 6:55 PM
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cracklover wrote: rgold wrote: [img]http://usera.ImageCave.com/rgold/Technical/Anchor.png[/img] This is, as claimed earlier, a version of the so-called American Death Triangle, a configuration that, while not ideal, isn't nearly as bad as its name suggests in many situations. This being one of them. This anchor is nothing to freak out over if the bolts are reasonable. But still, it isn't ideal; for the situation pictured, in-line clove hitches would have made much more sense. My physics is extremely rudimentary, so forgive me if I'm missing something obvious. But one implication of your equation above is that any time the two angles are equal, the force on the anchor is exactly L. Intuitively, this seems a little odd, since one could imagine a standard ADT anchor in which those two angles were equal (an equilateral triangle would do it) and in which we know that both bolts definitely see a force > L. GO
That is why it's bad. We have two bolts that both get the full load on the anchor. If you build a two piece anchor properly, each bolt should get half of the load.
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redlude97
Aug 11, 2010, 6:58 PM
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cracklover wrote: rgold wrote: [img]http://usera.ImageCave.com/rgold/Technical/Anchor.png[/img] This is, as claimed earlier, a version of the so-called American Death Triangle, a configuration that, while not ideal, isn't nearly as bad as its name suggests in many situations. This being one of them. This anchor is nothing to freak out over if the bolts are reasonable. But still, it isn't ideal; for the situation pictured, in-line clove hitches would have made much more sense. My physics is extremely rudimentary, so forgive me if I'm missing something obvious. But one implication of your equation above is that any time the two angles are equal, the force on the anchor is exactly L. Intuitively, this seems a little odd, since one could imagine a standard ADT anchor in which those two angles were equal (an equilateral triangle would do it) and in which we know that both bolts definitely see a force > L. GO
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jt512
Aug 11, 2010, 6:59 PM
Post #124 of 140
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redlude97 wrote: When discussing the forces on the pulley itself the forces are magnified relative to the input force.
I still think that is a silly way to think about the problem. Whatever weights you apply to either a fixed pulley or a bolt, the force on the pulley or bolt is still just a function of those weights. And if I interpret Rudmin's linked video correctly, then the force in the pulley case never be more than the force in the bolt case, and it will be less when the weights are unequal. So it seems to me that a single fixed pulley, if anything, is a force diminisher.
Jay
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milesenoell
Aug 11, 2010, 6:59 PM
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When I picture a pulley with 100 lbs on one side and 200 lbs on the other, I imagine the 100 lbs dangling and the 200 lbs on the ground, thus 200 lbs on the anchor.
If the rope were longer both objects are at rest and who cares about the pulley, and if the rope were shorter and the 100 lbs stops at the top before the 200 lbs hits the ground, then it's not acting as a pulley anymore. Thus, by elimination the only "pulley effect" we could apply here would be the scenario I was picturing in the beginning.
It may be incorrect, but it's my kind of answer.
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