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Ed Hartouni
Trad climber
Livermore, CA
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I have a mechanical model of a rope that I have to bust out on your guys...
...later tonight
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Mighty Hiker
Social climber
Vancouver, B.C.
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Topic Author's Reply - Jul 2, 2008 - 06:08pm PT
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If you want, I have a REAL rope that you can use for experiments. 60 m x 10.5 mm, somewhat used.
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tolman_paul
Trad climber
Anchorage, AK
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I'm glad to have had my last physics class 20 years ago. Just thinking about what would be needed to model this makes my head hurt.
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Mighty Hiker
Social climber
Vancouver, B.C.
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Topic Author's Reply - Feb 25, 2009 - 03:37pm PT
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Then there's this one - outstanding business.
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Mighty Hiker
Social climber
Vancouver, B.C.
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Topic Author's Reply - Mar 20, 2009 - 02:13am PT
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Still interested in the answer. There's a readable summary of the Goriely paper that Ed refers to near the start of the thread at http://www.spacemart.com/reports/Whip_Cracking_Mystery_Explained.html
Analyzing whips, concluding that a main factor allowing them to go supersonic at the tip is that they're tapered. Although unlike climbing ropes, which passively fall, whips are also given energy by hand motions.
Anyway, it does seem an interesting question. I wonder if it may take both theory and empirical results to determine the answer?
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Brock
Trad climber
RENO, NV
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Mar 20, 2009 - 10:43am PT
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I was repelling from Dolt once and I think it was my static line (could have been my lead line too though) but I remember it making a super loud "crack!" It does and can happen. So, if I am farting and it is loud, is it supersonic or is it just my butt cheeks are that tight???
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Greg Barnes
climber
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Jul 15, 2009 - 07:03pm PT
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When we were replacing bolts on Rock Warrior on the Velvet Wall after finishing up Dream of Wild Turkeys and Prince of Darkness, I would rap down a 100m 11m static, replace an anchor, rap to the next anchor, replace and fix the line, then they'd drop the rope and the other 3 guys would replace the pro bolts on the way down. The Chief was down at the base warning people off of the routes (good thing since we dropped a few hangers). One team decided to ignore The Chief and go up anyway on Prince of Darkness.
The guys 2 anchors up dropped the rope (I had a backpack to cover myself with as I hugged the rock), and the 100m static went zipping by, then made a louder-than-a-high-powered-rifle "crack" that echoed up and down Black Velvet Canyon. The core popped out as well.
But that was only a few feet away from the leader on Prince - I wonder if he actually crapped his pants or not...
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Mighty Hiker
Social climber
Vancouver, B.C.
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Topic Author's Reply - Jul 16, 2009 - 03:23pm PT
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The phenomena that wes mentions is interesting - it suggests that one possibility is the core of a kernmantle rope separating from the sheath, perhaps due to mass, density and other considerations, and literally 'popping' off the melted end. The cores of ropes do shift within the sheath, and they have different characteristics. Greater density suggests greater energy, which if directed along the axis of the rope could explain it. The core essentially blows the end off the rope.
I suspect a theoretical analysis of this problem may be complicated by the rope not being a single uniform body.
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Ed Hartouni
Trad climber
Livermore, CA
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OK, Anders really really wants an answer to this... so I've been thinking a bit about it for a while and will try (really try it is not so easy to do) to come up with an answer... but just to wet your appetites here is a picture of a rope at the bottom of the drop:
the numbers are the "strobe" flash, this was set up to be 24 flashes, each 1/30 of a second apart. This was set up pretty quickly just to see what I could do with my SB-28 Speedlight and the Fujifilm S5-Pro... came out ok...
what you see is that the distance from strobe 3 to 4 and strobe 4 to 5 is very big compared with the other inter strobe distances, which is the rope end accelerating.
Given a rope constituent model it may be possible to calculate the equation of motions and see just how fast the rope end can go under various drop conditions. It is possible that the rope acts like a whip, and the crack is the rope end exceeding the speed of sound. That shock wave could put forces on the rope tip that exceed the strength of the material...
...but I don't know yet. Anyway, there seems to be a hint of a path in getting to a reasonable solution.
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Mighty Hiker
climber
Vancouver, B.C.
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Topic Author's Reply - Sep 4, 2010 - 12:13am PT
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Thanks, Ed! A blast from the past, as it were. Although combined with yesterday's reapparance of the "History of Tube Chocks" thread, it may cause people to start thinking I'm some sort of geek wannabe.
It looks like you've set up an experiment, with falling ropes and strobe photography and so on. Thanks - a lot of effort. It may still be fun to do some empirical testing with a radar gun, to see what happens. Get some old ropes, someone a couple pitches up something quite overhanging, and let fly.
Nice to see that Juan was interested in this one, too.
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Shack
Big Wall climber
Reno NV
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That's cool Ed!
So what is the formula?
Terminal velocity + acceleration calculation = tip speed?
I'm sure it is waay more complex but just wondering the general idea.
I'm thinking that since I can easily make a towel tip break the sound barrier with very little energy input by me (I only have to flick my wrist), that the speed of the rope as it falls is only a small part of the equation, and that the way the end section of rope acts, as it reaches the bottom, is a far greater factor. The more rope that falls will affect the speed at which it falls which helps set up the needed conditions for the whip crack to occur and probably can't happen with a short rope. Rope diameter could be a factor as well.
My $.02
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Ed Hartouni
Trad climber
Livermore, CA
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the way to do the calculation is to have a model of how the rope acts as a solid, I have a model, it's pretty interesting and I'll write it up for this thread...
then you consider the rope as composed as segments that are attached like the model says they should be
constraining one end of the rope to a point in space let the other end fall in a uniform gravitational field in a fluid with finite viscosity, etc, etc... (the "fluid" is the atmosphere)
the trajectory for every point on the rope can be calculated, and the velocity of the rope's tip determined
but I'm a bit of a ways from that right now...
...probably no need for a "supercomputer" as the iBook I'm typing this message out on is a pretty good supercomputer by the standards of my graduate student experience (I think the microwave in the kitchen has a more powerful CPU then the one I used to do the data analysis for my thesis)...
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Shack
Big Wall climber
Reno NV
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Would you do different models with different weight per meter, maybe change diameters and see how it changes the outcome?
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Ed Hartouni
Trad climber
Livermore, CA
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once I get there different things can change to see what the effect of the differences would be...
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Paul_in_Van
Trad climber
Near Squampton
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MH,
Deceleration is the same as negative acceleration. Both positive and negative accelerations are just changes in velocity over time, but in different directions, either faster or slower.
P
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Mighty Hiker
climber
Vancouver, B.C.
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Topic Author's Reply - Sep 4, 2010 - 01:41am PT
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Physicists might not use the terms "deceleration" or "negative acceleration" at all. You simply state the acceleration, the vector of acceleration, the time, and the initial velocity (speed + direction), and that's all that's needed. (At least in Newtonian mechanics.)
Popular usage, trained by a century of motor vehicles, is acceleration and deceleration. It may not be precise in the terms of physicists, but is comprehensible.
I'll just go out to the car now, and negatively accelerate it.
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Shack
Big Wall climber
Reno NV
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First you must drive a negative distance.
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Ed Hartouni
Trad climber
Livermore, CA
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we're all Bozos on this bus [squeek, squeek]
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Mighty Hiker
climber
Vancouver, B.C.
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Topic Author's Reply - Sep 4, 2010 - 01:59am PT
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Hmmm, two climbing-geek related threads attributable to me on the front page. Things may be a bit anti-positive. Bwahahaha!
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bmacd
climber
Relic Hominid
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We can't be certain the rope cracking in this part of the universe is the same as in remote regions of space, If the fine-structure constant really does vary through space, it may provide a way of studying the elusive “higher dimensions” that many theories of bigfoot predict, but which are beyond the reach of particle accelerators on Earth.
http://economist.com/node/16930866?fsrc=scn%2Ffb%2Fwl%2Far%2Flawsofphysics
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