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patto
Feb 2, 2006, 4:22 AM
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WOW :)
I guess it wasn't a standing jump then :)
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misanthropic_nihilist
Feb 2, 2006, 4:49 AM
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In reply to: Are you absolutely certain that nylon webbing isn't linearly elastic?
Yes. It's definitely not linearly elastic under slackline tension. A material is linearly elastic when the ratio of Stress:Strain is constant. The easiest way to see that webbing doesn't hold to this is by tensioning your line, then coming back to it an hour later (or after bouncing on it). Although there is still the exact same amount of webbing stretched between your two anchors (strain is still the same), there is less tension (stress has decreased).
Simple Example:
Time 1:
Tension = 900 lbf
Strain = 10%
Time 2:
Tension = 700 lbf
Strain = 10%
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coldclimb
Feb 2, 2006, 5:46 AM
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In reply to: WOW :) I guess it wasn't a standing jump then :)
Hmm, good thing I stumbled on this randomly. Yeah that's a standing jump. I know nothing of the physics involved, but on a really tight line, I'd guess I can almost reach six feet. I can't land them that high though, very often. :wink: :lol:
Edit: now I've read over some of the discussion, and I see you're talking about a flat ground jump. That I've never measured, so I couldn't say. but I can hit an eight foot ceiling with my head from a standing jump, and I'm only six-two.
The physics I've never been too concerned with though. :wink:
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gymslackerclimber
Feb 2, 2006, 9:15 AM
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[quote="coldclimb"] Edit: now I've read over some of the discussion, and I see you're talking about a flat ground jump. That I've never measured, so I couldn't say. but I can hit an eight foot ceiling with my head from a standing jump, and I'm only six-two./quote]
gosh darn there coldclimb,,,, that is pretty hard core :!:
well my longest long jump was about 17ft my sophomore year,,,, :) :)
i think that is considered not so great...
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gymslackerclimber
Feb 2, 2006, 9:17 AM
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oh,,
i wonder how high you can kick,,?
i bet at least 8 and a half
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eric_t83
Feb 2, 2006, 1:10 PM
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In reply to: In reply to: Are you absolutely certain that nylon webbing isn't linearly elastic? Yes. It's definitely not linearly elastic under slackline tension. A material is linearly elastic when the ratio of Stress:Strain is constant. The easiest way to see that webbing doesn't hold to this is by tensioning your line, then coming back to it an hour later (or after bouncing on it). Although there is still the exact same amount of webbing stretched between your two anchors (strain is still the same), there is less tension (stress has decreased). Simple Example: Time 1: Tension = 900 lbf Strain = 10% Time 2: Tension = 700 lbf Strain = 10%
But that's because it changes over time. What I'm wondering about is if it's linear over a short period of time. A jump only lasts a couple of seconds, so maybe it behaves linearly over that short period.
The data supplied earlier in the thread by BlueWater seems to agree fairly well with this(I plotted the data in Matlab together with a linear best fit curve):
http://home.online.no/...igen/linetension.png
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misanthropic_nihilist
Feb 2, 2006, 2:20 PM
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Nice job with the graph. It looks like you might be right about the webbing being linear over a short time period.
Unfortunately, we're still stuck with the problem of the Modulus of Elasticity being largely variable. I'm not sure how variable it really is, but whenever I get the time I'd love to take a bunch of different brands, colors, ages, etc of webbing to the lab and play with them.
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veganboyjosh
Feb 2, 2006, 5:55 PM
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In reply to: I'd love to take a bunch of different brands, colors, ages, etc of webbing to the lab and play with them.
take a number...
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iltripp
Feb 2, 2006, 6:22 PM
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In reply to: But that's because it changes over time. What I'm wondering about is if it's linear over a short period of time. A jump only lasts a couple of seconds, so maybe it behaves linearly over that short period. The data supplied earlier in the thread by BlueWater seems to agree fairly well with this(I plotted the data in Matlab together with a linear best fit curve): http://home.online.no/...igen/linetension.png
I think that graph is slightly misleading... It suggests that webbing is linearly elastic up to 30% elongation. I doubt that is the case.
Instead, the initial elastic modulus is probably represented in the initial linear section of the material (approx 0-15% elongation).
One more thing, your graph would be a little more clear (at least to me) if you switched your axes. As far as I know load vs. elongation is usually the standard, not elongation vs. load.
I'll look back through the thread and see if I can find the data to play with.
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iltripp
Feb 2, 2006, 6:54 PM
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Ok... to add to the post above, I found the data and made my own graph. I switched the axes and made the dimensions such that the lack of linearity was more evident.
http://www4.ncsu.edu/~iltripp/curve.JPG
I threw a quick line in there to show the initial linear section (technically, it should go through the origin, but I'm lazy). This is where you would find the initial elastic modulus of the webbing. After that point, it is no longer linear.
Check out the picture below of polyester stress/strain curves. Although the material is different and will behave differently, you can see how a basic curve should look.
http://www.engr.utk.edu/...fiber_files/fig4.jpg
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zoratao
Feb 2, 2006, 7:08 PM
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1. As far as the outstanding jumps go, an outdated record for a standing high jump is about 4 ft. That was in the eighties by a professional basket ball player. You can idealize the model my taking the center of mass of the jumper to be the center of mass when standing, the change in height will be the change in height of the center of mass. Since the legs and the feet if extended fully during the jump are approximately rigid. Then it seems that one or two feet for the athletic person of non-olypian status is feasible. I could dunk a basketball in years done by and I am only 6'2" I have a long ape index so my extended height is around 8' and change. (Palming a basketball and what not. That puts me at my best vertical jump around three feet. (These are all approximations)
If timed right the jump from a non-static surface will give much greater effects.
2. As far as the leash fall issues, would it be possible to approximate the time it takes to stop a fall using video. This would at least give a fairly accurate impulse (and force for the falling slackliner) I think that time would be proportional to the time that the slackline retards the slacker during the jump. Are the quality processes for tubular webbing controlled tightly enough to give you batch dependant modulus'. It seems that if the nylon were consistent enough the webbing would be as well? Im not an engineer but I am interested in all things stress and strain (at least lately).
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eric_t83
Feb 3, 2006, 6:00 PM
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Iltrip, thanks for posting that. That's the problem with data analysis, you often find what you're looking for... :oops:
What does A,B,C,D and E stand for in the polyester graph?
Even if it's not linear, though, as long as the curve is known, you can find a dynamic solution. The problem is that a result from one batch of webbing probably wouldn't be comparable to another...
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iltripp
Feb 3, 2006, 7:11 PM
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In reply to: Iltrip, thanks for posting that. That's the problem with data analysis, you often find what you're looking for... :oops:
Very true...
In reply to: What does A,B,C,D and E stand for in the polyester graph?
I took a quick look at the website I pulled the graph from. They appear to be different types of polyester fibers and filaments... they probably have varying degrees of crystallinty as well as other differences.
In reply to: Even if it's not linear, though, as long as the curve is known, you can find a dynamic solution. The problem is that a result from one batch of webbing probably wouldn't be comparable to another...
I think that a finite, accurate calculation would be very difficult to obtain. Variations among batches of webbing as well as variations caused by wear, age, and weather would make any calculations inaccurate for determining real world conditions. However, that doesn't mean that we can't get in the ballpark.
Unfortunately, I'm not exactly sure how to approach the problem. If we were only dealing with low load/elongation (the somewhat linear portion of the curve), it would be reasonable to assume that the behaviour would be Hookean.
However, a session of backyard slacklining shows that it is not. For starters, webbing rather quickly undergoes what's referred to as stress decay (or stress relaxation). This is why you usually have to tighten a line after it's been in use for 15-20 minutes.
So, even if you could assume a mostly elastic response (and I don't know if that assumption is valid) to a single jump, a jump shortly after that would probably have a different elastic response.
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