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drfelatio
Feb 26, 2008, 3:17 PM
Post #26 of 34
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cracklover wrote: Drfelatio, you do realize that rgold posted the same in far more elegant math right here in this very thread? I also posted that, even earlier, but it's less offensive to me that most folks didn't seem to read or understand my post. What *really* bugs me is if folks can't be troubled to read RG's posts. He obviously puts real effort into crafting them - but if noone gives a damn, of course he'll stop posting here.
Someone woke up on the wrong side of the bed.
My intention wasn't to offend you or rgold. If anything, I was trying to reinforce your arguments because people kept saying "adding slack always increase FF" even after you and rgold showed otherwise.
I value rgold's input. His posts are always insightful and informative. I just found Jay's explanation easier to understand and thought others might too.
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cracklover
Feb 26, 2008, 3:42 PM
Post #27 of 34
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drfelatio wrote: cracklover wrote: Drfelatio, you do realize that rgold posted the same in far more elegant math right here in this very thread? I also posted that, even earlier, but it's less offensive to me that most folks didn't seem to read or understand my post. What *really* bugs me is if folks can't be troubled to read RG's posts. He obviously puts real effort into crafting them - but if noone gives a damn, of course he'll stop posting here. Someone woke up on the wrong side of the bed. My intention wasn't to offend you or rgold. If anything, I was trying to reinforce your arguments because people kept saying "adding slack always increase FF" even after you and rgold showed otherwise. I value rgold's input. His posts are always insightful and informative. I just found Jay's explanation easier to understand and thought others might too.
Ah, gotcha. If you'd said that before, I'd have had no issues. Anyway, it's all good!
I think that at this point that if folks don't get it, only mulling it over and doing the math for themselves (and maybe asking questions if they get stuck) will help.
One last thing - some folks have said that this stuff is non-intuitive. I'd disagree. Intuition simply means easily understandable based on deep knowledge of a subject. If folks take the time to really think about it, these things (fall factor and such) *do* begin to become more and more intuitive.
GO
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diebetes
Feb 26, 2008, 5:05 PM
Post #28 of 34
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I think this is what people get confused about: isn't there a difference between fall factor and impact force?
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skyski02
Feb 26, 2008, 6:23 PM
Post #29 of 34
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Sorry I posted the wrong picture. That was for a 20ft fall. Ooopsies. Here it is for a 15 ft fall. Same overall effect though.
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Attachments:
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FF_15_Corrected.JPG
(39.9 KB)
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skyski02
Feb 26, 2008, 6:23 PM
Post #30 of 34
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hugepedro wrote: skyski02 wrote: Thinking this over some more I would have to agree with that. If there is not much rope friction to 'slowly' release the slack and add to the dynamic breaking force . . . Actually, friction in the system can increase the force felt by the falling climber. The reason is that friction prevents the entire length of the rope from stretching immediately upon taking the load of the fall, so you don't get the full elongation capacity of the rope working its dynamic magic. The effect is as if the length of rope between belayer and climber has been shortened, thus higher forces.
What you are describing would be as if a kink in the rope caught somewhere and shortened the effective length of the rope. When friction on the rope kicks in before the climber reaches the low point in his fall it means it is applying a breaking force to the climber's fall which would not have normally existed. This effectively reduces the force the climber's fall exerts on the rope which is why you have reduced elongation since elongation is directly related to the force you apply to the rope.
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rgold
Feb 26, 2008, 7:01 PM
Post #32 of 34
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cracklover wrote: What *really* bugs me is if folks can't be troubled to read RG's posts. He obviously puts real effort into crafting them - but if noone gives a damn, of course he'll stop posting here.
Ho ho, thanks for the nice words, Gabe. Let's not forget I'm a college professor, which means I'm used to large numbers of people paying absolutely no attention to me whatsoever. The twisted character flaws that got me into this profession also endow me with both a very high tolerance for being ignored and an enormous capacity to enjoy the connection with the small minority who are actually interested in what I have to say. So...I'll probably keep posting even if almost no one seems to be listening.
It is true that the expression I wrote down for the change in fall factor shows, in a single line, that the change is positive for fall factors less than 1 (added slack increases fall factor) and negative for fall factors greater than one (added slack decreases fall factor, and also that the change is zero for fall factors equal to 1 (adding slack to a factor 1 fall leaves the fall factor unchanged). This certainly has the appearence of elegance, but one should note that I suppressed the derivation.
The derivation only involves high-school algebra, and illustrates that the manipulations you learn in high-school algebra can actually be used in the service of insight, a lesson that is sadly missing from many high-school curricula.
For those who want to dust off their high-school algebra, manipulate the change in fall factor expression
(H+S)/(L+S) - H/L
to get the "elegant" expression I posted.
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hugepedro
Feb 29, 2008, 11:51 PM
Post #33 of 34
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skyski02 wrote: hugepedro wrote: skyski02 wrote: Thinking this over some more I would have to agree with that. If there is not much rope friction to 'slowly' release the slack and add to the dynamic breaking force . . . Actually, friction in the system can increase the force felt by the falling climber. The reason is that friction prevents the entire length of the rope from stretching immediately upon taking the load of the fall, so you don't get the full elongation capacity of the rope working its dynamic magic. The effect is as if the length of rope between belayer and climber has been shortened, thus higher forces. What you are describing would be as if a kink in the rope caught somewhere and shortened the effective length of the rope. When friction on the rope kicks in before the climber reaches the low point in his fall it means it is applying a breaking force to the climber's fall which would not have normally existed. This effectively reduces the force the climber's fall exerts on the rope which is why you have reduced elongation since elongation is directly related to the force you apply to the rope.
#1 Me thinks you didn't read my post too carefully, see above bolded word.
#2 A kink or a pinched rope would certainly be a couple situations that could cause this, but they are not the only mechanisms by which friction can cause higher forces.
#3 Friction is not applying a braking force that would not normally have existed, it is merely extending the duration of the fall.
#4 Friction could reduce the ultimate forces generated, or it could increase them, which is why I said can above.
#5 Most the time it doesn't even matter, and we shouldn't waste any energy thinking about it, unless our top piece is sketchy.
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t2stone
Mar 2, 2008, 10:46 AM
Post #34 of 34
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