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asuclimber
Dec 4, 2005, 6:57 PM
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It seems there are two schools of thought on placing marginal pro that has a good chance of pulling in a fall:
1. Pro that rips out slows you down, reducing the force felt by your next piece
2. Pro that rips out wastes the rope's dynamic stretching ability, putting a more static, and thus higher load on your next piece.
Which is the more important factor 1, 2, or is the reality somewhere in between?
In another thread, alpnclmbr1 argued that the force on the second piece is increased as the rope has lost some of its stretching ability:
In reply to: For example: climber 5ft x pro 10ft x first fall w/ stretch and at point of failure of the top piece 15ft fall 10ft x 10ft belay 10ft stopping point of second fall, total fall 30ft =-=-=--=-=-= first fall = 10ft fall / 35 feet of rope = .3 FF assume 5 feet of rope stretch before failure of the last piece second fall 20ft fall – 35 feet of rope = .57 FF add in the excess energy left over from the original fall and the loss of energy absorption capability in the rope from the original fall and the force would be that much greater.
In alpnclmbr1's scenario, if the climber didn't place that first piece, the force on the second piece would be that of a ~ff 1.14 (30 ft falling, 10 ft rope stretch, on 35 ft of rope).
With both pieces in place, alpnclmbr1 argues that the "excess energy left over from the original fall and the loss of energy absorption capability in the rope from the original fall" makes the second piece feel a force greater than it would feel without the first piece altogether. In order for the force on the second piece(assuming a ripped first piece) to exceed the force felt on a fall on that piece alone, the excess energy and loss of rope stretch from the first fall would have to double the fall factor.
.57 ff (on alpnclmbr1's second piece) x 2 (due to excess energy and loss of rope stretch) = 1.14 (ff on 2nd piece alone)
To me, the loss of stretch and excess energy doubling the force on a piece seems high.
The bottom line: Will pulling a piece of marginal pro increase or decrease the force on your next piece of pro?
Also, how is it related to the pro spacing?
Ex 1: blowing 2 rp's within a foot of a nut vs Ex 2: blowing 2 rp's 10' and and 5' above a nut
Ex 1:---------------------------------------Ex 2:
climber-------------------------------------climber
15'------------------------------------------5'
x rp-----------------------------------------x rp
x rp-----------------------------------------5'
x nut(3 pieces within 1 ft)-------------------x rp
10'------------------------------------------5'
x--------------------------------------------nut
10'------------------------------------------10'
Belay----------------------------------------x
100'-----------------------------------------10'
Deck----------------------------------------Belay 100' to deck
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scottquig
Dec 4, 2005, 7:08 PM
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The leader should also consider that putting in marginal gear takes time, makes the leader more tired, and it wastes gear that you might potentially need later in the pitch. This doesn't mean that I'm neccessarily in favor of running stuff out...just something to consider.
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stymingersfink
Dec 4, 2005, 7:43 PM
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^^True, but sometimes pro you think would not hold a fall- DOES.
I think marginal pro comes in two categories. Those in which the placement is marginal, and those in which the limitations of the gear may be suspect.
I would rather trust a piece of well placed gear with a rating <4kn if it were the only thing I could get in, over a piece with a higher rating but sketchy placement. In such a scenario, the use of a screamer would be indicated.
The thing to remember is that a piece of gear pulling out happens so quickly, that its net reduction in fall force is negligible and some postulate may reduce the elasticity of the rope (?).
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stymingersfink
Dec 4, 2005, 7:48 PM
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..and too, check the photo sequence of the guy on Ruby's Cafe pulling gear (in the "ever pull a cam" thread, i think). What do you think he might say when asked such a question... I'd bet he didn't even know he was above marginal pro!
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iltripp
Dec 4, 2005, 7:57 PM
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You mean these:
Ouch!!!
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overlord
Dec 4, 2005, 8:11 PM
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:shock: :shock: :shock: :shock: :shock: :shock:
was he seriously injured??
im just guessing... pro was marginal because hes use of it was.
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asuclimber
Dec 4, 2005, 8:41 PM
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Can anyone answer this:
Will pulling a piece of marginal pro increase or decrease the force on your next piece of pro?
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iltripp
Dec 4, 2005, 8:50 PM
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In reply to: Can anyone answer this: Will pulling a piece of marginal pro increase or decrease the force on your next piece of pro?
No....
I can't answer that.
I've seen it debated ad nauseam, but never seen any definitive evidence to support either case. Perhaps someone out there has some data...
It would definitely be an interesting thing to test. With the right equipment, it seems like an easy experiment to design.
Any equipment manufacturers wanna hire me to do some research?
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rgold
Dec 5, 2005, 12:07 AM
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I don't think we know the answer to this for sure, but the expert opinions I've heard lean towards the position that pulling pieces can reduce impacts further down. As asuclimber points out, alpenclimber1's post, at least the part asuclimber reproduces, doesn't appear to address the issue. The question is whether the impact on the second piece that would occur if there was no first piece will be any smaller if there is a first piece that fails.
The question boils down to how much the rope stretches to extract the piece (which is determined by the amount of load the piece can handle) and the recovery the rope experiences in the time between a piece pulling and the impact on the next piece. The more complete the recovery, the more fall energy the rope can remove while stretching before the piece pulls and then "reset" when the failure happens. Some time ago, (9/5/2000) rec.climbing physics guru Ken Cline argued that a significant recovery probably does happen:
In reply to: Some relaxation will take place as fast as the shock wave can travel the rope, l/v (where v is the speed of sound an l the rope length). On the other hand, we all know that ropes need to relax for minutes (hours? days?) to regain their full energy absorbing potential. The ultimate answer is somewhere between quite fast and forever. First, let's consider the fast side of the coin: As any physicist knows, the speed of longitudinal waves in a string is given by v = sqrt(F/mu), where F, the rope tension, might be in the 2-10 kN range, and mu, its mass per unit length, is around .07 kg/m. So v is around 150-400 m/s. Approximately. Even at the slower speed, the relaxation wave will travel, say 15 meters of rope in 0.1 second. That's pretty fast - certainly faster than you will fall (terminal velocity is on the order of 50m/s). I think it is safe to assume a good deal of relaxation occurs in practice, even if we include factors like friction which I left out for simplicity.
This position was, at some point, echoed in the programming of the Petzl fall calculator, which allowed one to specify pulled gear (I don't know whether this is still the case). It was clear from the results that, at the time, the Petzl calculator assumed a complete rope recovery after gear pulled.
Other suggestive pieces of evidence for recovery are the graphs of some of the Italian computer-modeled falls, which are shown to correlate well with experimental results. In the graph that does not involve belaying off the harness, there is something like a 40% recovery or bounce in the 0.8 sec after the rope achieves maximum extension. This is a weighted recovery in which the leader is pulled back up, not the presumably greater weightless recovery that would occur when a piece pulls.
Of course, the fact that the rope makes a significant recovery in length does not mean that it makes an analogous recovery in energy-absorbing capability. It might be "stiffer" than you would predict from the length recovery. If it was a lot stiffer, than it is possible that even though it had recovered in length and even though some fall energy had already been absorbed, the absorbtion of the remaining energy by a much stiffer rope could result in a higher load on the second piece than if the leader had just fallen directly on to it. This question can probably only be settled by experiment.
In the absence of definitive experimental results, it is still not unreasonable to assume that pulling gear can lower impact loads further down. Moreover, it seems to me to be impossible to argue that a load on a lower piece would be higher by virtue of an upper piece pulling than it would have been without the upper piece---without postulating an increase in stiffness (as measured by the rope modulus). If there is no recovery, then the total amount of rope stretch is the same for the fall with the pulled piece and the fall without the pulled piece, because the two falls are the same height with the same amount of total rope out, the same energy must be absorbed, and rope stretch is the energy absorbtion mechanism. Since rope tension is more or less proportional to rope stretch, the lower piece gets the same impact either way if there is no recovery and rope modulus does not increase.
So it seems to me that as far as we know, the extra gear probably won't hurt in terms of impacts lower down, and might help. Whether the possibly small effect of marginal gear is worth the energy drain the climber endures to place it is a whole n'other quite situational question.
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iltripp
Dec 5, 2005, 12:23 AM
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Wow...
Thanks rgold. Always a pleasure to read your posts.
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iltripp
Dec 5, 2005, 12:33 AM
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In reply to: Of course, the fact that the rope makes a significant recovery in length does not mean that it makes an analogous recovery in energy-absorbing capability. It might be "stiffer" than you would predict from the length recovery. If it was a lot stiffer, than it is possible that even though it had recovered in length and even though some fall energy had already been absorbed, the absorption of the remaining energy by a much stiffer rope could result in a higher load on the second piece than if the leader had just fallen directly on to it. This question can probably only be settled by experiment.
Any chance you can explain a few points for me:
It seems to me that significant recovery in length would be analogous with energy-absorbing capability, unless the basic material properties were changed by the fall.
In other words, are you saying that after a fall the modulus of the material is temporarily changed (raised I suppose)?
Does it then recover it's normal modulus over time?
What kind of time frame are we talking about here (you mentioned hours or days)?
Thanks
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chossmonkey
Dec 5, 2005, 4:21 AM
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So the guy ripping his gear out of Ruby's walked away with bruised ribs.
Was he just lucky or did the gear, even though it pulled, slow him down some?
In case anyone is wondering the crack is supposed to take #3 Metolius and he was plugging #2 size.
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rgold
Dec 5, 2005, 5:21 AM
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In reply to: It seems to me that significant recovery in length would be analogous with energy-absorbing capability, unless the basic material properties were changed by the fall. In other words, are you saying that after a fall the modulus of the material is temporarily changed (raised I suppose)?
Yes. I believe this has been documented, but I'm sorry to say I don't have a reference. In fact, consider the mechanism that causes the rope to break after a certain number of UIAA falls. Why does it break? After all, the fall factors are the same. I assume it breaks because the modulus goes up, so the tension involved in absorbing the same quantity of fall energy goes up, and eventually this tension exceeds the tensile strength of the rope.
Another situation that may be caused by rope stiffening is the failure of pro that has successfully held one or more falls. Most of us know of several such incidents. The failure is usually blamed on shifting of the pro or deterioration of the rock, but it seems to me that another possibility is that a stiffer rope develops higher tension when arresting the same fall, and the higher load is more than the pro can withstand.
I believe that this possibility is why some rope manufacturers have recommended changing ends of the rope when taking repeated falls.
In reply to: Does it then recover it's normal modulus over time?
To some extent, but probably not completely. An interesting question is how fast the modulus increases with use. I've never heard anyone say authoritatively. Of course, quantifying "use" is problematic. We do know that the number of UIAA falls held goes down with use, and I suspect this means the modulus is going up---if you buy the breaking explanation above.
In reply to: What kind of time frame are we talking about here (you mentioned hours or days)?
Well actually the original question had to do with fractions of a second. I have heard figures between fifteen minutes and half an hour mentioned for a "substantial" recovery, but I don't know how authoritative those claims were either. I have also heard more than once that washing a used rope helps to recover its original modulus through some process of "shrinking" the fibers. This too seems to belong to the realm of folklore and I don't have any sources to point to for confirmation.
Any engineers out there who know about any of these aspects of rope behavior?
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iltripp
Dec 5, 2005, 5:50 AM
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In reply to: Yes. I believe this has been documented, but I'm sorry to say I don't have a reference. In fact, consider the mechanism that causes the rope to break after a certain number of UIAA falls. Why does it break? After all, the fall factors are the same. I assume it breaks because the modulus goes up, so the tension involved in absorbing the same quantity of fall energy goes up, and eventually this tension exceeds the tensile strength of the rope.
That seems to make sense. I'd love to see some data on this, though. I would like to know how much the modulus of the rope is affected over time and how much recovery actually takes place.
In reply to: Another situation that may be caused by rope stiffening is the failure of pro that has successfully held one or more falls. Most of us know of several such incidents. The failure is usually blamed on shifting of the pro or deterioration of the rock, but it seems to me that another possibility is that a stiffer rope develops higher tension when arresting the same fall, and the higher load is more than the pro can withstand. I believe that this possibility is why some rope manufacturers have recommended changing ends of the rope when taking repeated falls.
Interesting. I'm sure that pro shifting is part of the problem (as this is something that I have seen and heard of happening), but maybe the higher tension is also a factor. Thanks again for taking the time to write this.
In reply to: In reply to: Does it then recover it's normal modulus over time? To some extent, but probably not completely. An interesting question is how fast the modulus increases with use. I've never heard anyone say authoritatively. Of course, quantifying "use" is problematic. We do know that the number of UIAA falls held goes down with use, and I suspect this means the modulus is going up---if you buy the breaking explanation above.
It seems like it would be a fairly easy thing to measure by following a standard UIAA fall test with a tensile test to determine modulus. After the next fall test, another modulus measurement could be made.
In reply to: In reply to: What kind of time frame are we talking about here (you mentioned hours or days)? Well actually the original question had to do with fractions of a second. I have heard figures between fifteen minutes and half an hour mentioned for a "substantial" recovery, but I don't know how authoritative those claims were either.
Again... this seems like a relatively simple thing to test, at least with the 15 minutes to half an hour theory. The partial recovery that occurs between pieces failing (i.e. the original question) would be difficult to quantify, but making tensile measurements at successive intervals after a UIAA fall test should tell us exactly what kind of recovery goes on within minutes to hours of a fall.
In reply to: I have also heard more than once that washing a used rope helps to recover its original modulus through some process of "shrinking" the fibers. This too seems to belong to the realm of folklore and I don't have any sources to point to for confirmation. Any engineers out there who know about any of these aspects of rope behavior?
Hmmm... I am not an engineer, but I am a polymer chemist (or at least I will be in about 10 days when I get my diploma) and this seems pretty fishy to me. While there are many interesting things that go on chemically when a rope is wet, I fail to see how a noticeable recovery of mechanical properties could occur due to washing. Water is a plasticizer for Nylon, which will definitely have some effects on the rope's mechanical properties (including decreasing the modulus), but these effects should only be present while the rope is actually wet.
Man... I would love to get involved in this kind of testing. Maybe I can get into an engineering program somewhere and work with ropes, etc.
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misanthropic_nihilist
Dec 5, 2005, 6:12 AM
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In reply to: Man... I would love to get involved in this kind of testing. Maybe I can get into an engineering program somewhere and work with ropes, etc.
I opened up a can of worms a while ago asking about the properties of rope....
http://www.rockclimbing.com/...hp?p=1202523#1202523
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misanthropic_nihilist
Dec 5, 2005, 6:13 AM
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I would think that having marginal gear pull would always decrease the maximum force felt by the piece that finally catches you.
Short Explanation:
The rope should return to it's unstretched length quickly enough that it can readily absorb the impact from the next piece of gear.
Long Explanation:
Whether the rope regains it's energy absorbing (i.e. goes back to its unstretched length) between when a piece pulls and when the next piece takes the load, think of it this way: Say it took 150 pounds to pull out a piece. That means there was about 75 pounds of tension in the rope (due to a 2:1 mechanical advantage on the piece by the rope). As soon as that piece pulls, you have the equivalent of letting go of rubber band. Since the rope is no longer supporting your weight at all, that entire 75 pounds of force goes into unstretching the rope. If that amount of rope weighs, say, 7.5 pounds, the rope is going to unstretch at approximately 10 times the acceleration due to gravity. Basically, the rope returns to it's original length pretty quickly, probably faster than you're falling.
Now, every piece that rips out will allow the rope to stretch, absorb some of your energy, then unstretch. Each failure point will absorb some energy, ultimately decreasing the force felt by the final piece.
(Note: The above statements and calculations assumed a lot of idealizations. I ignored many things, including: air resistance, the mass of the piece of protection, the change in rope tension as stretch decreases, etc)
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iltripp
Dec 5, 2005, 6:17 AM
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In reply to: As soon as that piece pulls, you have the equivalent of letting go of rubber band. Since the rope is no longer supporting your weight at all, that entire 75 pounds of force goes into unstretching the rope.
Hmm... I think you are assuming that a Nylon rope exhibits perfectly elastic behaviour (like a rubber band). It doesn't.
Now, I have a hunch that falling on marginal pro does reduce the final force on subsequent pieces, but according to what rgold has said, we really don't know how quickly the rope recovers it's ability to absorb energy.
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misanthropic_nihilist
Dec 5, 2005, 6:23 AM
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In reply to: In reply to: As soon as that piece pulls, you have the equivalent of letting go of rubber band. Since the rope is no longer supporting your weight at all, that entire 75 pounds of force goes into unstretching the rope.Hmm... I think you are assuming that a Nylon rope exhibits perfectly elastic behaviour (like a rubber band). It doesn't.
At the instant the piece pulls, my assumption should be correct. Once the rope starts to contract, all bets are off. There's no reason to do the exact integral calculations involved since there are so many unaccounted for variables.
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overlord
Dec 5, 2005, 6:35 AM
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it does reduce the force regardless of rope behaviour.
why??
because you need some force to rip pro and that force comes from you falling. thus the speed of your fall is reduced because some energy was used for ripping pro.
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karlbaba
Dec 5, 2005, 6:37 AM
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Just to throw a few practical bits in here:
First, the purpose of placing marginal gear is because you hope it will actually catch you. That is often a chance worth taking. I've had a rurp catch a 25 footer on the Shield.
Second, it's true that one bad fall can compromise the rock around the piece under the right (wrong) circumstances. I took a 30 footer on a #1 wired hex places sideways after a few pieces ripped including breaking the cable on a #4 stopper. When I got back to the piece to finish the lead, the rock around the piece shattered when I cleaned it.
Third, there are a lot of folks who aren't accurate enough judges of the viability of what's marginal to make the call. I'd beware of suggesting not leaving a piece out of worry that it will affect pieces further down. In reality, most pieces will either rip right out without stressing the rope much, or catch the fall.
Fourth, There is a fourth but I can't think of it.
Fifth, numbered lists are arbitrary
Peace
karl
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pk
Dec 5, 2005, 8:43 AM
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In reply to: So the guy ripping his gear out of Ruby's walked away with bruised ribs. Was he just lucky or did the gear, even though it pulled, slow him down some? In case anyone is wondering the crack is supposed to take #3 Metolius and he was plugging #2 size.
Is this 100% correct? The wrong gear was in fact the cause for this fall?
P.K.
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j_ung
Dec 5, 2005, 2:13 PM
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That the guy in iltripp's pics walked with bruised ribs would also seem to argue in favor of failing pieces reducing impact force. From the photos, it's clear how he landed. Given the same landing, but nothing to slow the fall, would he have walked with such a minor injury? I can think of a few factors that might affect the outcome either way, but intuitively, I lean toward the marginal piece helping more than hurting.
But the equally important question is how likely am I to fall if I expend energy to place the marginal piece? Can I make it to a better placement/stance? It's the classic pro-or-go dillemma, with the added variable of marginal pro. Gotta love those split-second life-or-death decisions. :?
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kubi
Dec 5, 2005, 3:39 PM
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Overlord is right, a peice of pro ripping out is going to absorb some energy no matter what, probably very little, maybe a lot. Either way you are better off then you were before.
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chossmonkey
Dec 5, 2005, 6:45 PM
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In reply to: In reply to: So the guy ripping his gear out of Ruby's walked away with bruised ribs. Was he just lucky or did the gear, even though it pulled, slow him down some? In case anyone is wondering the crack is supposed to take #3 Metolius and he was plugging #2 size. Is this 100% correct? The wrong gear was in fact the cause for this fall? P.K.
This link is with the pictures in the original posting on RC.com.http://www.cascadeclimbers.com/...age/0/gonew/1#UNREAD
It is as true as what was posted there. One cam was a #2 Metolius and the other was another brand of the equivalent size. It's anybodies guess how tipped out they were. A #2 in a tight #3 placement in really hard rock would probably be okay.
Back to the question at hand. Think about a Screamer. It doesn't absorb energy by stretching as a rope does. It absorbs by many tiny failures in the stitching. Not that this is the same, but I thought I would throw that out for some thought.
It seems like this could be answered pretty quickly with testing. As sexy as calculations are, there are so many variables, testing would probably be more accurate.
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granite_grrl
Dec 5, 2005, 7:09 PM
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I believe the answer is somewhere in between. Pulling a piece would certainly absorb some energy....but the rope will loose some of its stretch and its ability to absorb energy immediately after.
I think its a question of how far you will fall before you encounter your next piece. After one piece pulls and you fall a little further your velocity will certainly increase.
Therefore it is a matter of:
- how much force the piece that pulled absorbed from the initial portion of your fall
- how the rope's ability to absorb force has decreased
- the speed you are falling and how much force it will take to bring you to a resting state
There is also the question of how quickly the rope will regain its stretch, but in my musings I have taken this to be negligible when falling from one piece to the next.
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