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jt512
Jan 17, 2006, 5:38 AM
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In reply to: In reply to: In reply to: Jay: The problem I have with just giving slack is that the system isn’t analogous to the figure skater who increases her angular velocity while twirling by bringing her arms closer to her body. In other words, she’s in a situation in which angular momentum is conserved. Not so for the climber. Giving slack causes the climber to fall further down the cliff before hitting wall. As an example, it there’s a meter of rope out, which is held tight, then when the climber bangs into the wall, his body needs to dissipate the potential energy from falling vertically 1 meter. If slack is provided so that 2 meters of rope is out, his body needs to dissipate the energy from falling 2 meters. (Obvious simplification in ignoring such items as energy absorption by rope, etc. Just trying to make point that energy is being put into the system so that angular momentum is not conserved.) I’m not saying one shouldn’t give out slack, but rather it’s a complicated problem, which most likely would call for different actions in different situations. Cheers, RobKelman.calm 16-Jan-06 21:24:00 MST (-6 UMT) Pnedulum fall: give slack. It really is that simple, though I'm sure the collective internet community will have no trouble coming up with several pathological counter-examples. Jay No, it's not that simple--it's situational. Feed out slack and mgh increases. Curt
But so does the amount of rope out to absorb the additional energy. The result is that letting rope out slightly increases the fall factor, but greatly decreases the horizontal velocity. It is pretty simple: you have a rope to limit the impact force with the rope, and a rock face to limit the impact force with the wall. If y'all put your heads together, I'm sure you can proudly name many exceptions, but in general, it should be clear that the rock wall presents the greater danger, and is the impact you want to take steps to minimize.
Jay
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curt
Jan 17, 2006, 5:58 AM
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In reply to: In reply to: In reply to: In reply to: Jay: The problem I have with just giving slack is that the system isn’t analogous to the figure skater who increases her angular velocity while twirling by bringing her arms closer to her body. In other words, she’s in a situation in which angular momentum is conserved. Not so for the climber. Giving slack causes the climber to fall further down the cliff before hitting wall. As an example, it there’s a meter of rope out, which is held tight, then when the climber bangs into the wall, his body needs to dissipate the potential energy from falling vertically 1 meter. If slack is provided so that 2 meters of rope is out, his body needs to dissipate the energy from falling 2 meters. (Obvious simplification in ignoring such items as energy absorption by rope, etc. Just trying to make point that energy is being put into the system so that angular momentum is not conserved.) I’m not saying one shouldn’t give out slack, but rather it’s a complicated problem, which most likely would call for different actions in different situations. Cheers, RobKelman.calm 16-Jan-06 21:24:00 MST (-6 UMT) Pnedulum fall: give slack. It really is that simple, though I'm sure the collective internet community will have no trouble coming up with several pathological counter-examples. Jay No, it's not that simple--it's situational. Feed out slack and mgh increases. Curt But so does the amount of rope out to absorb the additional energy. The result is that letting rope out slightly increases the fall factor, but greatly decreases the horizontal velocity. It is pretty simple: you have a rope to limit the impact force with the rope, and a rock face to limit the impact force with the wall. If y'all put your heads together, I'm sure you can proudly name many exceptions, but in general, it should be clear that the rock wall presents the greater danger, and is the impact you want to take steps to minimize. Jay
Right. Then, by that logic, the amount of rope you should feed out (to improve the situation) should be infinite.
Curt
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cintune
Jan 18, 2006, 7:48 PM
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But only if you have infinite space to fall in.
Fallllllliiiiinnggggggggggggggggggggggggggggggggg................
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billl7
Jan 18, 2006, 8:19 PM
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There are two aspects in this slack-or-not-to-slack for a pendulum fall:
more slack -> more potential energy
more slack -> smaller percentage of energy is converted into horizontal motion
Anyone care to whip out the pencil and see which one dominates (edit: in terms of total horizontal energy)?
Bill
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robdotcalm
Jan 18, 2006, 8:39 PM
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[quote="billl7"
more slack -> more potential energy
more slack -> smaller percentage of energy is converted into horizontal motion
Bill
How did you arrive at the 2nd conclusion?
Cheers,
Rob.calm
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billl7
Jan 18, 2006, 8:52 PM
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In reply to: In reply to: more slack -> more potential energy more slack -> smaller percentage of energy is converted into horizontal motion Bill How did you arrive at the 2nd conclusion? Cheers, Rob.calm
The varying angle makes a difference in how much of the total energy is transferred into horizontal energy. But also total energy is higher when there is slack. And so was wondering which one made more difference in terms of the final horizontal energy.
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pastprime
Jan 18, 2006, 9:33 PM
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Good info, thanks. It occurs to me that in an alpine/trad situation where it is hard to communicate with your belayer, if you were leading and in danger of a pendulum into a wall, it would be wise to be pulling slack as you went, and the amount of slack you would want to keep in the rope would be enough that your vertical drop before hitting the end of the rope would be somewhat more than the distance of the swing horizontally that would follow.
Example: your last piece is at the top of an open book, and you have led 10 feet horizontally to the right without yet being able to get anything in. you would be wise to have pulled enough rope so that there is at least 15 feet of rope between you and the last piece, and better more than that than less.
Do you physicists agree, or am I seeing it wrong?
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talons05
Jan 18, 2006, 9:57 PM
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In reply to: Good info, thanks. It occurs to me that in an alpine/trad situation where it is hard to communicate with your belayer, if you were leading and in danger of a pendulum into a wall, it would be wise to be pulling slack as you went, and the amount of slack you would want to keep in the rope would be enough that your vertical drop before hitting the end of the rope would be somewhat more than the distance of the swing horizontally that would follow. Example: your last piece is at the top of an open book, and you have led 10 feet horizontally to the right without yet being able to get anything in. you would be wise to have pulled enough rope so that there is at least 15 feet of rope between you and the last piece, and better more than that than less. Do you physicists agree, or am I seeing it wrong?
I am not a physicist, but have studied a lot of physics for my major. IMHO, what you say does have a basis in classical physics. But you have to find a compromise. As the length of rope increases relative to your horizontal distance from the piece, the angle of your fall increases (steepens) and approaches (but does not reach) vertical. Of course at some point, this fails to help and begins to increase the force again. Probably somewhere around 1.5x the horizontal distance.
Basically, you are trying to increase the y (vertical) component of your trajectory since that is the vector in which the rope is able to absorb force.
Cheers,
A.W.
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billl7
Jan 18, 2006, 10:09 PM
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In reply to: Good info, thanks. It occurs to me that in an alpine/trad situation where it is hard to communicate with your belayer, if you were leading and in danger of a pendulum into a wall, it would be wise to be pulling slack as you went, and the amount of slack you would want to keep in the rope would be enough that your vertical drop before hitting the end of the rope would be somewhat more than the distance of the swing horizontally that would follow. Example: your last piece is at the top of an open book, and you have led 10 feet horizontally to the right without yet being able to get anything in. you would be wise to have pulled enough rope so that there is at least 15 feet of rope between you and the last piece, and better more than that than less. Do you physicists agree, or am I seeing it wrong?
I'm no physicist but the interplay isn't hard to calculate when assuming a very simple system (e.g., not addressing how friction and/or stretch come into play differently). I'll try to do it tonight if someone doesn't beat me to it; either way it would be good for us to have more than one person weigh in on the issue.
Bill
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jt512
Jan 18, 2006, 10:09 PM
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None of the arguments so far, except mine, have taken into account that even though you produce more energy with more slack out, you also have more rope out to absorb that energy. The net result, in practical situations, is a only a small increase in the vertical component of impact force, so small that you probably wouldn't notice it. I haven't done the math, but the small increase in impact force is a small price to pay for the reduction in horizontal swing.
Jay
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curtis_g
Jan 18, 2006, 10:20 PM
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In reply to: In reply to: In reply to: What's the best way to take that kind of fall? I just sprained the bejesus out of my ankle taking a pendulum fall. I don't remember exactly the specifics but I'm guessing I tried to absorb some of the shock with my legs and did it wrong. Any suggestions? What is not intuitively obvious is that the force involved is exactly the same as a vertical fall. What force, the impact force? The impact force is much higher for a pendulum fall into a wall than it is for a vertical fall onto a rope. In reply to: That's why intentional payout of slack may sometimes be a good idea. Changes the directon of fall to a more conventional angle. Slack reduces the impact force into the wall because it increases the radius of the arc and hence reduces the angular velocity. -Jay
Jay, you have a lot of things wrong here with what actually happens.
First, the impact of swinging into a wall is exactly the same as hitting the ground if you were to be fallign vertical the same distance as that of the rope that has been paid out to allow for the swing.
Second, the payout of slack does help but keeping slack in the system would not help. If you think about the translation of vertical fall velocity into a horizontal velocity, initial slack in the system would act like the climber taking a larger swing. The tricky, dangerous, and highly skilled payout of slack during a fall would, in fact, lessen the inpact on the slab you would be swinging into.
To experience this hang a weight by a string, give it some inches slack and drop it at the top of a pendulum. By the time it crosses a plum line from your hand to the ground, it is moving faster than no slack. (make sure yo take the slack from your hand and not just move the climber closer to the swing point.) Then try, as your 'climber' starts to swing in, drop your hand and notice how much slower he climber crosses that plum line.
That's the physics of the pendulum, well not really why, just what happens (form a mech. eng. major), but that gradual payout is difficult, and sometimes painfull, for the belay. My advice would be...
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.
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billl7
Jan 18, 2006, 10:20 PM
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In reply to: None of the arguments so far, except mine, have taken into account that even though you produce more energy with more slack out, you also have more rope out to absorb that energy. The net result, in practical situations, is a only a small increase in the vertical component of impact force, so small that you probably wouldn't notice it. I haven't done the math, but the small increase in impact force is a small price to pay for the reduction in horizontal swing. Jay
Bill
Edit: curtis_g - I don't see anything wrong with your quotes of Jay, allowing for some interpretation on my part. Maybe we can hold off with the debate until someone produces some numbers? (no abuse intended)
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pastprime
Jan 18, 2006, 10:27 PM
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It also occurs to me, which thing I had never before considered, that if you are faced with a traversing exit from a corner, you would be better off not placing that last piece high in the corner before leaving, which is a pretty common habit. It seems it would be better to get a good piece into the corner a ways below the exit so your progress is more decidedly above the piece than sideways from it.
Never thought of that before. Good to know.
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cracklover
Jan 18, 2006, 10:33 PM
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In reply to: None of the arguments so far, except mine, have taken into account that even though you produce more energy with more slack out, you also have more rope out to absorb that energy. The net result, in practical situations, is a only a small increase in the vertical component of impact force, so small that you probably wouldn't notice it. I haven't done the math, but the small increase in impact force is a small price to pay for the reduction in horizontal swing. Jay
Another point:
If you give the classic version of a soft catch* you can "absorb" a fair bit of energy through the friction in the rope around the bends over the top biner, belay biner, and belay device.
GO
*Do people know what a soft catch even means? It doesn't mean just feeding out slack before the rope goes tight. It means allowing some rope to slip through the belay device as it comes tight. Practice.
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pastprime
Jan 18, 2006, 10:39 PM
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OK, I did the weight on a string, holding one end against the wall, and letting it drop from a point horizontally away from the anchor. With no slack, it really smacks the wall, just as expected. With even a bit of slack in the line before dropping, the impact is quite dramatically reduced.
Try it you own self. You'll see.
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pastprime
Jan 18, 2006, 11:21 PM
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I shouldn't have said "even a bit of slack", that's an exaggeration; the crossover point where an improvement is pretty apparent seems to be where the dropping object falls a distance of half or more of the horizontal distance it was from the anchor before it comes tight on the string. I would expect on a dynamic rope where much of the energy is dissipated before being translated to horizontal motion, the difference would be even greater.
I hate trying to write coherently here at work, where there's not time to think out the proper way to say things before consigning them to all the world/
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curtis_g
Jan 18, 2006, 11:28 PM
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In reply to: In reply to: None of the arguments so far, except mine, have taken into account that even though you produce more energy with more slack out, you also have more rope out to absorb that energy. The net result, in practical situations, is a only a small increase in the vertical component of impact force, so small that you probably wouldn't notice it. I haven't done the math, but the small increase in impact force is a small price to pay for the reduction in horizontal swing. Jay Another point: If you give the classic version of a soft catch* you can "absorb" a fair bit of energy through the friction in the rope around the bends over the top biner, belay biner, and belay device. GO *Do people know what a soft catch even means? It doesn't mean just feeding out slack before the rope goes tight. It means allowing some rope to slip through the belay device as it comes tight. Practice.
This is what I was talking about as the gradual feed of slack during the fall. It does need to be practiced esp. by new climbers that tend to get fingers all up in the friction and ouch, bad news and possibly a drop. It pretty much came naturally to me, no one ever taught me it so I didn't know what to call it but I have always done out of personal comfort mostly.
To the reply after the one i quote: I know what you are talking about when you are seeing less of a horizontal velocity with initial slack, but make sure your 'anchor' hand that the 'rope' is swinging from is fixed and you will also notice a 'jarring' from a change in direction. Climbing ropes, being dynamic, will transfer the direction of the force much smoother and eliminate this jarring, but with the smoother transfer in this direction of force, comes a smoother transfer in the direction of velocity. The fall won't hurt as much in your transfer to horizontal motion because of the rope dynamics, but it will hurt more in the acceleration retained by a smooth motion transfer...an equal payoff.
Think about the scenario of initial slack in the rope/system in this mindest. For the first few feet of the pendulum, you are in a freefall until the slack is picked up. Now that you have fallen you have gained vertical speed which will translate into higher horizontal speed when you hit the wall on your swing.
Or...after the slack has been taken up by a fall, picture yourself still on the rock, but only with your anchor point 'magically' moved above its original position by a few feet. You will be lower in relation to your anchor but will have more rope paid out to allow you to swing during the fall....yet another equal payoff.
Again, the best idea: forethought...have your belay practice soft catches with youand you practice too...out of courtesey. Haha.
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curtis_g
Jan 18, 2006, 11:39 PM
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In reply to: I shouldn't have said "even a bit of slack", that's an exaggeration; the crossover point where an improvement is pretty apparent seems to be where the dropping object falls a distance of half or more of the horizontal distance it was from the anchor before it comes tight on the string. I would expect on a dynamic rope where much of the energy is dissipated before being translated to horizontal motion, the difference would be even greater. /
After reading some more I found this.
yes, a 'great amount' of slack will allow the stretch of the rope to perform just like the 'soft catch' method that I described. It wold just be the amazing amount of rope and its porportional amount of stretch that would be providing the 'soft' part. The only problem or more like the only possibly problematic factor would be the affordable amount of rope available on each spicific climb.
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cintune
Jan 19, 2006, 1:37 PM
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In reply to: It pretty much came naturally to me, no one ever taught me it so I didn't know what to call it but I have always done out of personal comfort mostly.
That's a good point. It's definitely something you have to have a feel for, whether or not you want to talk about forces and vectors and calculations. It's a matter of the belayer deliberately manipulating the rope to create an elastic moment between the initial fall and the swing. In the end, though, smacking into a wall is smacking into a wall. How it turns out is more up to the faller than the belayer, if it's up to anyone. Having a soft catch provides an extra second or two to get your reflexes primed for what's about to happen, while a tight pendulum might be over before you know it.
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billl7
Jan 19, 2006, 3:27 PM
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Did get through some numbers last night but not ready to post. Was using the model equivalent to a steel cable, no soft catch, and the only slack is past the last piece. Either I have something wrong or it does not pay in practice to have additional slack for that unrealistic model. Hope to look at it more tonight.
Anyone else look at the numbers or have ideas about modeling the rope stretch and/or soft catch?
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jt512
Jan 19, 2006, 4:35 PM
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In reply to: In reply to: None of the arguments so far, except mine, have taken into account that even though you produce more energy with more slack out, you also have more rope out to absorb that energy. The net result, in practical situations, is a only a small increase in the vertical component of impact force, so small that you probably wouldn't notice it. I haven't done the math, but the small increase in impact force is a small price to pay for the reduction in horizontal swing. Jay Right. Your point is relevant: there's more rope to stretch but also the fall factor goes up as slack is paid out. But I was thinking of starting with a simple system - maybe fall factor could be brought in later or maybe not if the pay-off for slack is clear enough (assuming it doesn't cause a collision with something else). Bill
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.
Jay
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billl7
Jan 19, 2006, 5:01 PM
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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.
What I mean by "capture" would be to have an outcome that says, the model shows X benefit in slack and things like rope stretch and soft catch will only make it better. By the way, for those promoting the soft catch, how much rope is typically let out in a "soft catch"?
Bill
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daithi
Jan 19, 2006, 5:09 PM
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In reply to: In reply to: In reply to: In reply to: In reply to: Jay: The problem I have with just giving slack is that the system isn’t analogous to the figure skater who increases her angular velocity while twirling by bringing her arms closer to her body. In other words, she’s in a situation in which angular momentum is conserved. Not so for the climber. Giving slack causes the climber to fall further down the cliff before hitting wall. As an example, it there’s a meter of rope out, which is held tight, then when the climber bangs into the wall, his body needs to dissipate the potential energy from falling vertically 1 meter. If slack is provided so that 2 meters of rope is out, his body needs to dissipate the energy from falling 2 meters. (Obvious simplification in ignoring such items as energy absorption by rope, etc. Just trying to make point that energy is being put into the system so that angular momentum is not conserved.) I’m not saying one shouldn’t give out slack, but rather it’s a complicated problem, which most likely would call for different actions in different situations. Cheers, RobKelman.calm 16-Jan-06 21:24:00 MST (-6 UMT) Pnedulum fall: give slack. It really is that simple, though I'm sure the collective internet community will have no trouble coming up with several pathological counter-examples. Jay No, it's not that simple--it's situational. Feed out slack and mgh increases. Curt But so does the amount of rope out to absorb the additional energy. The result is that letting rope out slightly increases the fall factor, but greatly decreases the horizontal velocity. It is pretty simple: you have a rope to limit the impact force with the rope, and a rock face to limit the impact force with the wall. If y'all put your heads together, I'm sure you can proudly name many exceptions, but in general, it should be clear that the rock wall presents the greater danger, and is the impact you want to take steps to minimize. Jay Right. Then, by that logic, the amount of rope you should feed out (to improve the situation) should be infinite. Curt
Absolutely. As the rope length goes to infinity the horizontal component of velocity goes to zero (presuming you had an infinite space to fall in!) :)
In general I would agree though and say more slack would reduce the horizontal component of velocity.
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daithi
Jan 19, 2006, 5:21 PM
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In reply to: Right. Your point is relevant: there's more rope to stretch but also the fall factor goes up as slack is paid out. But I was thinking of starting with a simple system - maybe fall factor could be brought in later or maybe not if the pay-off for slack is clear enough (assuming it doesn't cause a collision with something else).
The closer the rope angle is to vertical when it is weighted in a pendulum fall the more kinetic energy it absorbs and the horizontal velocity is less. Therefore the energy absorption of the system is a crucial component.
Describe the fall you are trying to model and I will put some numbers on it.
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billl7
Jan 19, 2006, 5:35 PM
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In reply to: In reply to: Right. Your point is relevant: there's more rope to stretch but also the fall factor goes up as slack is paid out. But I was thinking of starting with a simple system - maybe fall factor could be brought in later or maybe not if the pay-off for slack is clear enough (assuming it doesn't cause a collision with something else). The closer the rope angle is to vertical when it is weighted in a pendulum fall the more kinetic energy it absorbs and the horizontal velocity is less. Therefore the energy absorption of the system is a crucial component. Describe the fall you are trying to model and I will put some numbers on it.
* climber is 10 feet into horizontal traverse from last piece of pro;
* the point at impact is vertically below the last piece of pro;
variable:
* the amount of slack on the climber's side of last piece of pro;
my model constraints:
* no rope stretch (albeit a significant omission);
* kinetic energy from vertical fall translates to KE in pendulum motion through unsophisticated trig properties at the point where the rope goes taught;
* from pendulum begin (rope taught) to pendulum end, total KE is increased by the vertical height change through the actual pendulum (not including any initial vertical fall)
So, the leader's horizontal KE is the component derived from the KE of the vertical drop plus the KE from the vertical change during the pendulum.
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