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.

That's why intentional payout of slack may sometimes be a good idea. Changes the directon of fall to a more conventional angle.

[
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.

-Jay

I'm getting a refresher course in physics!

Curt, I fell parallel to the wall, was just starting to move vertically (but at an angle) off the traverse when my feet slipped. I remember trying to bring my body around so I could absorb some of the shock with my legs but hit the wall too soon. I definitely spun once or twice. I must have braced with my (now) injured ankle but not in ANY kind of controlled fashion Those are good tips. I think my biggest mistake was not planning for the possibility of that fall. Like the graytradster said, I think I got off lightly.

Slack reduces the impact force into the wall because it increases the radius of the arc and hence reduces the angular velocity.

Curt, I was leading (woohoo! Sorry, still new, obviously, and excited about it!) and fell back towards the last bolt. I have a nice (rope?) burn on my arm from it too. I think I was so focused on trying to control the fall I didn't "go with it" and thus was more injured. Patting along the wall might have been more scary but might have ended up better. It's better now though...I can actually bear weight now!

I've seen several references to Arno and his clinics. If I do a search will I get numerous threads with relevant info?

:roll:

Technically, Jay, it wasn't the angular velocity that hurt his ankles. It was the tangential linear velocity. Or, to be more precise, it was the sudden, rapid deceleration from that linear velocity that caused his pain. :wink:

Responses composed with the intent to project maximum apparent intellect always sound so patronizing.

(side note to crackrn -- he's right, though)

As your post clearly demonstrated.


And therefore the point of your post was, what, then?

-Jay

That "patting along" idea is how I describe it, cause I patted my way down (mostly vertical fall) a high angle slab once and didn't get a scratch. It was like playing high speed patty cake with your hands and feet, sort of, and it worked.

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

How did you arrive at the 2nd conclusion?

Cheers,
Rob.calm

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?

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?

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.


Slack reduces the impact force into the wall because it increases the radius of the arc and hence reduces the angular velocity.

-Jay

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

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.

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.
/

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.

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.

Right. Then, by that logic, the amount of rope you should feed out (to improve the situation) should be infinite.

Curt

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.

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

how much rope is typically let out in a "soft catch"?

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

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

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

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

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.

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

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.

that's crazy that you did put numbers/a model to it in an experiment.

You don't need to quantify it at all. You need only take a 10-foot fall onto a static rope.

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.

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.

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.

- initial slack makes negligable (in theory, if not zero) difference

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

I still question some of the conclusions made for not leaving slack. Only because my basic logic says differently, ....

If it makes no difference ito you in theory, you need to find another theory.

Jay

v_tangent = [sqrt(2Gh)] * [L/(sqrt(h^2 + L^2)]

But so does the amount of rope out to absorb the additional energy.

The point is only the component of linear momentum that is absorbed by the rope is the component that is parallel to the rope (it would be the dot product of the momentum vector and the vector that describes the rope direction). The rope has no mechanical means (or almost none) to absorb energy in any direction other than along it.

All the energy is still there! It can't just go away or 'be absorbed' like so many people have claimed. That's day one stuff. Actually its the law of conservation of energy. IT'S a LAW...you all should go to jail...or at least pay me a hefty fine.

you couldn't be more wrong. energy will escape the system.

as it seems the only way to reintroduce it to the climber's tangential velocity is if the rope significantly springs back well before the climber hits the wall.

for some reason i find it hard to accept when you say that the rope absorbs energy. it could gain a certain potential engery stored in its stretch and its stretch could slightly warm up the rope as energy will always escape into dissepated heat (negligable), and in a direct vertical fall many things (rope, belay, anything in the system including the two harnesses) are moved and tensioned as well as the climber springing back up as he bounces on the rope.

ok so i read on and you comment that I would readily believe that this does happen. as the climber nears a total horizontal vector of velocity, wouldn't the rope regain some of it's pull and toss the climber a bit harder...or possibly the climber would oscillate during his swing that would loot something like the function of y = x sinx (with any number of phase shifts or stretches determined by extraneous factors of the fall) as the climber falls from x = 2 pi in the negative direction towards x=0. just an example/thought.

Throw in a soft catch and there would be even less and weaker oscillations as well (where's Jay! he's probably going to miss another opportunity to crow!! :) ).

Bill

But while I *think* I do understand the more complex principles that come into play with the soft catch, I don't know how to plot them out, and of course I wouldn't have the means to test my theories either.

Actually, I introduced the soft catch way back on page 3. Unfortunately, curtis_g agreed with me, causing me to lose all credibility. :)

Describe how you think it works in the form of a differential equation, or failing that even the principle and I will do the maths and the numerical modeling part (at some stage!).

With a small amount of slack out, the climber falls further, generating more energy, and the faller's motion is transfered into swinging motion while still quite a bit sideways from the last piece, so the pull is not parallel to the rope, and the increased energy is still mostly absorbed by the climbers body smacking the wall.

With a whole lot of slack out at the time of the fall, even more energy is generated that is going to need to be absorbed at some time, but by the time the faller hits the end of the rope, the rope is much closer to being lined up with the direction the faller is moving, so much more of the energy is absorbed by the rope before or as translation to horizontal movement occurs.

I just have to know, how many of you posting in this thread lately have taken any big pendulums and done it safely ? cause from the way you are writing it sounds like you have not done it very much.

Dear Jay, I will restate...with initial slack in the system (your rope), you will only increase your vertical fall velocity and THEN translate it into the SAME AMOUNT of horizontal velocity

I don't know how many. Why wait for a response? Tell us what you think from experience.

Since the approach to mathematical modeling generally starts out fairly conservative, because most of us try the easiest things first, the approach could be confused with lack of experience. However, in my case you can accurately chalk it up to inexperience if you like. :)

Bill

I don't blame you for wanting to model, and it woud be interesting, but in the end, when you take the fall two things will matter: Are you in a good falling position and have you got a plan for meeting the wall?

My bad. :)

Which do you think typically causes more pendulum elongation: belay slippage or plain rope stretch? That probably needs some qualification but I'm too lazy right now.

I have trouble believing that the slope is so steep. It doesn't seem reasonable that a tiny increase in slack would increase the impact force by nearly 40%.

I may be misunderstanding you but ... multiply R by the amount of the horizontal traverse before the fall. Take that number and subtract the horizontal travese. This gets on the amount of slack added.

So the point on the line to the right of the spike where the line crosses "1" - that would be about one hundred and eighty percent of the horizontal traverse as added slack.