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jt512
Jan 19, 2006, 6:09 PM
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In reply to: In reply to: If you ignore the additonal rope in the system to absorb energy, then you've ignored a major factor, and might as well not do the problem. If you want to ignore something to simplify the problem, ignore the increase in the fall factor due to letting slack out, because it is minimal anyway. You probably know, the concept of fall factor does include additional rope stretch. But accounting for this in a model is hard. A conservative approach either takes into account or captures the things that help in terms of not smashing into the wall. Ignoring the increase in fall factor isn't conservative. Showing why it is negligible would be helpful. Bill
A model has to take into account all the salient factors. I don't see how you can ignore rope stretch.
Showing that the increase in fall factor resulting from slack is ignorable -- at least compared with assuming the rope doesn't stretch -- is simple. Assume you have 80 feet of rope out and the climber falls from 5 feet above top pro. With no slack, ff = 10/80 = .125. With 3 feet of slack, a realistic amount, ff = 13/83 = .157. In practice, the climber will not notice the difference beteen these fall factors, whereas the difference between assuming no stretch in the rope and using a realistic dynamic rope is the difference between life and death. So, if you've got to ignore something, ignore the increase in fall factor, rather than assume that the rope doesn't stretch.
Jay
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cintune
Jan 19, 2006, 6:12 PM
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Sort of like a fall on a zipline, even though it's static. There's the initial drop, which shortens the distance of the...zipping...part. Feeding slack on a dynamic rope lengthens the radius of the pendulum, but it shortens the arc traveled. Win-win situation.
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billl7
Jan 19, 2006, 6:14 PM
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In reply to: how much rope is typically let out in a "soft catch"?
Reason I ask is, if the amount of rope let out is as negligible as some say for the effect of purposefully added slack on fall factor then I will defend that a soft catch makes (editted for clarity here) little difference in the pendulum impact. Reasoning is that the soft catch lessons the jarring at the point where the rope goes taught but this effects only the force in-line with the rope and not the force associated with the climber's acceleration into the beginning of the pendulum.
Bill
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billl7
Jan 19, 2006, 6:32 PM
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In reply to: A model has to take into account all the salient factors. I don't see how you can ignore rope stretch.
This is only because you have prejudged that rope stretch is necessary and significant in terms of answering the question. "prejudged" is probably too strong of a word; maybe its just an issue of whether you have sufficiently justified that.
I'm ready to concede that fall factor does not change significantly. :)
Bill
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jt512
Jan 19, 2006, 6:52 PM
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In reply to: In reply to: A model has to take into account all the salient factors. I don't see how you can ignore rope stretch. This is only because you have prejudged that rope stretch is necessary and significant in terms of answering the question. "prejudged" is probably too strong of a word; maybe its just an issue of whether you have sufficiently justified that. I'm ready to concede that fall factor does not change significantly. :) Bill
Maybe I haven't read the posts in the thread clearly enough to understand the question. But it seems to me that the rope stretch changes everything, a lot. If it didn't, we would use static ropes.
Jay
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billl7
Jan 19, 2006, 6:54 PM
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I need to go back on what I said about the soft catch. The soft catch is going to contribute in the same way that rope stretch will. It delays the transfer of KE from the vertical fall into the KE that the climber will have going into the pendulum (when the rope goes taught). By delaying the transfer of the force the angle between vertical and the taught rope is allowed to decrease a bit more and so less KE from the vertical fall is transferred into the pendulum swing.
Sorry.
All this still doesn't answer the question of which one dominates:
more slack -> overall greater energy (regardless of what absorbs it);
more slack -> less percentage of energy from the vertical fall is transfered to horizontal;
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cintune
Jan 19, 2006, 6:56 PM
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In reply to: the soft catch lessons the jarring at the point where the rope goes taught but this effects only the force in-line with the rope and not the force associated with the climber's acceleration into the beginning of the pendulum.Bill
- That's it, and the pendulum itself describes a shorter arc, therefore less acceleration before impact. It's all good.
If the rope did stretch enough, you'd leave an upward-pointing red smear on the rock when it lifted what was left of you, under reduced load. :)
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billcoe_
Jan 19, 2006, 7:01 PM
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In reply to: Maybe I haven't read the posts in the thread clearly enough to understand the question. But it seems to me that the rope stretch changes everything, a lot. If it didn't, we would use static ropes. Jay
You could always try reading them J :wink: .
Sure it matters, point is, the farther you fall, the faster you go. (talking slow here). Even if it is sideways.
Then, if you happen to smack a wall at the end of you penji, the physics change even faster the other direction.
Better not to fall.
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cintune
Jan 19, 2006, 7:02 PM
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In reply to: the soft catch lessons the jarring at the point where the rope goes taught but this effects only the force in-line with the rope and not the force associated with the climber's acceleration into the beginning of the pendulum.Bill
- That's it, plus the pendulum itself describes a shorter arc, therefore less overall acceleration before impact. It's all good.
If the swing was long enough and the rope did stretch enough, you'd leave a nice upward-pointing red smear on the rock. :)
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billl7
Jan 19, 2006, 7:19 PM
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In reply to: Wear a helmet, don't close your eyes, keep your head over your heels, hold on for the ride, and don't poop yourself...or cry, and at least you'll have your dignity.
... just thought it was worthy of repeating ...
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curtis_g
Jan 19, 2006, 8:11 PM
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that's crazy that you did put numbers/a model to it in an experiment. Just like I explained, slack in the system after the the last piece of pro wouldn't help because while you start the swing farther along in the fall, you are also falling farther.
the dynamic rope and a dynamic belay catch (soft catch) would make the difference. the original person that suggested slack in the system revised his suggestion to say something like "i meant lots of slack" and I already commented on that practicality and how the rope would onyl help then in providing the "soft catch" instead of the experienced belay.
like you said, it really comes down to a) an experienced belay but more inportantly b) knowing how you are going to fall and keeping your head straight during the fall enough to control yourself
to model a soft catch i would say to have the rope run from the climber over a pulley and the other end of the rope attached to a weight maybe 1/4 the weight of your climber sitting on a ledge so that the 1/4 weight will be lifted off of it's ledge maybe halfway through the climber's fall.
or i would just use some shock cord like from the inside of a tent pole.
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jt512
Jan 19, 2006, 8:14 PM
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In reply to: In reply to: Maybe I haven't read the posts in the thread clearly enough to understand the question. But it seems to me that the rope stretch changes everything, a lot. If it didn't, we would use static ropes. Jay You could always try reading them J :wink: . Sure it matters, point is, the farther you fall, the faster you go. (talking slow here). Even if it is sideways.
Bill, the whole point is that even though you build up more speed/energy the further you fall, YOU HAVE A FUCKING DYNAMIC ROPE THAT ABSORBS MOST OF THE ADDITIONAL ENERGY, and thus negating it. Therefore, how can you dream of ignoring the rope? If you have to ignore something, ignore the additional energy, because the rope takes care of most of that.
Jay
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robdotcalm
Jan 19, 2006, 8:27 PM
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In reply to: In reply to: In reply to: Maybe I haven't read the posts in the thread clearly enough to understand the question. But it seems to me that the rope stretch changes everything, a lot. If it didn't, we would use static ropes. Jay You could always try reading them J :wink: . Sure it matters, point is, the farther you fall, the faster you go. (talking slow here). Even if it is sideways. Bill, the whole point is that even though you build up more speed/energy the further you fall, YOU HAVE A f---ing DYNAMIC ROPE THAT ABSORBS MOST OF THE ADDITIONAL ENERGY, and thus negating it. Therefore, how can you dream of ignoring the rope? If you have to ignore something, ignore the additional energy, because the rope takes care of most of that. Jay
The rope absorbs some additional energy. The real problem is to quantify if this is signficant or not.
rob.calm
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curtis_g
Jan 19, 2006, 8:28 PM
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that's crazy that you did put numbers/a model to it in an experiment. Just like I explained, slack in the system after the the last piece of pro wouldn't help because while you start the swing farther along in the fall, you are also falling farther.
the dynamic rope and a dynamic belay catch (soft catch) would make the difference. the original person that suggested slack in the system revised his suggestion to say something like "i meant lots of slack" and I already commented on that practicality and how the rope would onyl help then in providing the "soft catch" instead of the experienced belay.
like you said, it really comes down to a) an experienced belay but more inportantly b) knowing how you are going to fall and keeping your head straight during the fall enough to control yourself
to model a soft catch i would say to have the rope run from the climber over a pulley and the other end of the rope attached to a weight maybe 1/4 the weight of your climber sitting on a ledge so that the 1/4 weight will be lifted off of it's ledge maybe halfway through the climber's fall.
or i would just use some shock cord like from the inside of a tent pole.
...but there was some ongoing discussion started by someone who probably sounded to many like they were correct in their reasoning while reall they weren't. the comment was made that a soft catch, either by rope or the belay, will simply delay the transfer of Kinetic Energy of the vertical fall into the KE of th swing. First, the energy of the fall would be porportionate to the height of the fall, so if the climber is let fall a greater distance vertically by means of a soft catch then wouldn't less potential energy remain to translate into horizontal KE? Fact is, as more slack is paid out during the fall, the lesser the angle (vertex on the last piece of pro) of the rope. Now consider vectors and it should be clear to see that, with this lessened angle during the fall, more of the force of the fall will be felt vertically (how we are used to) and less will be translated into horizontal KE.
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jt512
Jan 19, 2006, 8:33 PM
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In reply to: In reply to: In reply to: In reply to: Maybe I haven't read the posts in the thread clearly enough to understand the question. But it seems to me that the rope stretch changes everything, a lot. If it didn't, we would use static ropes. Jay You could always try reading them J :wink: . Sure it matters, point is, the farther you fall, the faster you go. (talking slow here). Even if it is sideways. Bill, the whole point is that even though you build up more speed/energy the further you fall, YOU HAVE A f---ing DYNAMIC ROPE THAT ABSORBS MOST OF THE ADDITIONAL ENERGY, and thus negating it. Therefore, how can you dream of ignoring the rope? If you have to ignore something, ignore the additional energy, because the rope takes care of most of that. Jay The rope absorbs some additional energy. The real problem is to quantify if this is signficant or not. rob.calm
You don't need to quantify it at all. You need only take a 10-foot fall onto a static rope.
Jay
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jt512
Jan 19, 2006, 8:36 PM
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In reply to: that's crazy that you did put numbers/a model to it in an experiment. Just like I explained, slack in the system after the the last piece of pro wouldn't help because while you start the swing farther along in the fall, you are also falling farther.
*broken record* You have more rope out to absorb the additional energy. The fall-factor hardly increases. You have more rope out to absorb the additional energy. The fall-factor hardly increases. You have more rope out to absorb the additional energy. The fall-factor hardly increases. You have more rope out to absorb the additional energy. The fall-factor hardly increases. You have more rope out to absorb the additional energy. The fall-factor hardly increases. You have more rope out to absorb the additional energy. The fall-factor hardly increases. You have more rope out to absorb the additional energy. The fall-factor hardly increases. You have more rope out to absorb the additional energy. The fall-factor hardly increases. You have more rope out to absorb the additional energy. The fall-factor hardly increases. You have more rope out to absorb the additional energy. The fall-factor hardly increases. You have more rope out to absorb the additional energy. The fall-factor hardly increases. You have more rope out to absorb the additional energy. The fall-factor hardly increases.
Jay
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billl7
Jan 19, 2006, 8:38 PM
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In reply to: that's crazy that you did put numbers/a model to it in an experiment. |
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billl7
Jan 19, 2006, 8:46 PM
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In reply to: You don't need to quantify it at all. You need only take a 10-foot fall onto a static rope.
Bill
Editted to get the quote right.
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cchildre
Jan 19, 2006, 8:59 PM
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My single point. The angle of the pendlum can be decreased by paying out some extra slack. So if a climber falls on a tensioned rope at the same height of his last piece of pro and is ten feet out. His starting angle is 90 degrees and the fall would immediately pull him back towards his last pro. If he has say 5 feet of slack then he falls down 3-5 feet before he starts to load the rope and start the swing thus the starting angle would be more like 50 or 60 degrees. Their still swinging just not as violently and perhaps in a more controlled manner. Get some physics guys to explain it...but then we would probably still be scratching our heads.
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curtis_g
Jan 19, 2006, 9:21 PM
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In reply to: by cchildre My single point. The angle of the pendlum can be decreased by paying out some extra slack. So if a climber falls on a tensioned rope at the same height of his last piece of pro and is ten feet out. His starting angle is 90 degrees and the fall would immediately pull him back towards his last pro. If he has say 5 feet of slack then he falls down 3-5 feet before he starts to load the rope and start the swing thus the starting angle would be more like 50 or 60 degrees. Their still swinging just not as violently and perhaps in a more controlled manner. Get some physics guys to explain it...but then we would probably still be scratching our heads.
my point, however hidden it was, was that while this angle between the sections of rope on either side of the last piece of pro will be reduced, with slack in the system (but no soft catch), the advantage of the decreased angle will be negated by the increase of energy produced by that extra five feet of slack that will be 'used up' entirely in freefall. so...the speed of the swing would only be reduced by the release of slack during the fall. I think I worded this explination best in my reply one earlier.
even if you cant exactly undertand my explination, try my 'weight on a string' example in my first post so that you can see what works...initial slack doesn't. visually measure the speed by the height of the pendulum after it has past where the climber would normally go smack.
IN CONCLUSION...
- we all use dynamic ropes so end that argument already
- initial slack makes negligable (in theory, if not zero) difference
- practice your soft catches
- keep your head straight during the fall and don't poop yourself
- the outcome is most essentially a result of the climber's actions during the fall
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curtis_g
Jan 19, 2006, 9:23 PM
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In reply to: by cchildre My single point. The angle of the pendlum can be decreased by paying out some extra slack. So if a climber falls on a tensioned rope at the same height of his last piece of pro and is ten feet out. His starting angle is 90 degrees and the fall would immediately pull him back towards his last pro. If he has say 5 feet of slack then he falls down 3-5 feet before he starts to load the rope and start the swing thus the starting angle would be more like 50 or 60 degrees. Their still swinging just not as violently and perhaps in a more controlled manner. Get some physics guys to explain it...but then we would probably still be scratching our heads.
my point, however hidden it was, was that while this angle between the sections of rope on either side of the last piece of pro will be reduced, with slack in the system (but no soft catch), the advantage of the decreased angle will be negated by the increase of energy produced by that extra five feet of slack that will be 'used up' entirely in freefall. so...the speed of the swing would only be reduced by the release of slack during the fall. I think I worded this explination best in my reply one earlier.
even if you cant exactly undertand my explination, try my 'weight on a string' example in my first post so that you can see what works...initial slack doesn't. visually measure the speed by the height of the pendulum after it has past where the climber would normally go smack.
IN CONCLUSION...
- we all use dynamic ropes so end that argument already
- initial slack makes negligable (in theory, if not zero) difference
- practice your soft catches
- a WHOLE LOT of rope will act like a soft catch
- keep your head straight during the fall and don't poop yourself
- the outcome is most essentially a result of the climber's actions during the fall
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jt512
Jan 19, 2006, 9:44 PM
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In reply to: In reply to: You don't need to quantify it at all. You need only take a 10-foot fall onto a static rope. Jay, you may be correct about everything you've said but your powers of persuasion have room for improvement. Remember, a vertical fall as you suggest here is not the same as a pendulum because in a pendulum the rope simply cannot absorb all the energy. And mimicing a broken record has zero merit - indeed, negative merit. Bill Editted to get the quote right.
I'm not trying to pursuade anyone. Do what you want with your model, just don't use it to belay me, if your results are based on a static rope.
Jay
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jt512
Jan 19, 2006, 9:47 PM
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In reply to: - initial slack makes negligable (in theory, if not zero) difference
If it makes no difference ito you in theory, you need to find another theory.
Jay
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billl7
Jan 19, 2006, 10:32 PM
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In reply to: In reply to: In reply to: You don't need to quantify it at all. You need only take a 10-foot fall onto a static rope. Jay, you may be correct about everything you've said but your powers of persuasion have room for improvement. Remember, a vertical fall as you suggest here is not the same as a pendulum because in a pendulum the rope simply cannot absorb all the energy. And mimicing a broken record has zero merit - indeed, negative merit. Bill Editted to get the quote right. I'm not trying to pursuade anyone. Do what you want with your model, just don't use it to belay me, if your results are based on a static rope. Jay
Bill
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cchildre
Jan 19, 2006, 10:46 PM
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I still question some of the conclusions made for not leaving slack. Only because my basic logic says differently, and I lack the working knowledge to challenge any of it. So, unless I go study up, do the math, and conclude otherwise, I shall stand down. This all comes back to the individuals choice. I belay everyone according to their wishes. You want it tight, loose, hell you want a hip belay....I am the slave!
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