|
|
|
|
RobDude
Feb 8, 2009, 6:51 AM
Post #1 of 49
(3851 views)
Shortcut
Registered: Jan 20, 2009
Posts: 25
|
So, this is probably a retarded question. But it's been bugging me....
I'm about 180-185 pounds. My girlfriend is probably 125-130. How, exactly, is she able to belay me?!?
We climb indoors (so far) and we've always top-roped. The rope goes up to a large pipe, it's wrapped around once, and then back down. There is no pulley system, as far as I understand it, no mechanical advantage....if one end of the rope goes up 2 feet, the other end goes down 2 feet.
So, when I get to the top and am being lowered down; why doesn't she get lifted off the ground?
Again, obviously she doesn't and all - but I don't get it. My initial thought was that there is a significant amount of friction caused by the rope being wrapped around the pipe. Let's say '100 pounds' worth. So, my 185 - 100 = 85. And since she is > 85 pounds, she'd remain on the ground....but that doesn't make any sense either - because if it took 100 pounds of force to move the rope - she'd be unable to pull in the slack on the rope.
So - what gives?
(Yes, I'm a 'tard)
|
|
|
|
|
macblaze
Feb 8, 2009, 6:55 AM
Post #2 of 49
(3845 views)
Shortcut
Registered: Jun 23, 2005
Posts: 807
|
A little thing called friction. Try the same thing on a couple of carabiners and she'll definitely get some liftage...
As for the math... who knows. It just works.
(This post was edited by macblaze on Feb 8, 2009, 6:55 AM)
|
|
|
|
|
majid_sabet
Feb 8, 2009, 7:09 AM
Post #3 of 49
(3835 views)
Shortcut
Registered: Dec 13, 2002
Posts: 8390
|
(This post was edited by majid_sabet on Feb 11, 2009, 5:46 PM)
|
|
|
|
|
spoon
Feb 8, 2009, 7:36 AM
Post #4 of 49
(3812 views)
Shortcut
Registered: Aug 26, 2002
Posts: 312
|
Frictional force on the rope is determined in part by the magnitude of the normal force, or how hard it is pushing on the surface against which it is rubbing. Think of it like this. If you try to pull a sled across the ground it's pretty easy. If you try to pull the same sled full of rocks across the same ground it's much harder. When your girlfriend is pulling in slack, there is hardly any weight on the rope so there's hardly any friction, (that's the empty sled). When you are leaning back on the rope, there's lots of weight on the rope so there's a whole lot of friction (that's the sled full of rocks). I hope that helps.
|
|
|
|
|
irregularpanda
Feb 8, 2009, 9:15 AM
Post #5 of 49
(3779 views)
Shortcut
Registered: Mar 13, 2007
Posts: 1364
|
Fuck all that friction shit.
Wizards live in the pipe. Kepp them happy with sacrificial goats and do your happy dance.
|
|
|
|
|
0x2102
Feb 8, 2009, 1:53 PM
Post #6 of 49
(3728 views)
Shortcut
Registered: May 17, 2005
Posts: 115
|
Think of the pipe as a pulley - it effects a mechanical advantage of 2:1, so your 185 lb load feels like 92.5 lbs to the belayer. Add in a little friction and the load of the climber on the belayer is reduced even more.
|
|
|
|
|
adatesman
Feb 8, 2009, 2:01 PM
Post #7 of 49
(3724 views)
Shortcut
Registered: Jul 13, 2005
Posts: 3479
|
|
|
|
|
|
kirksullivan
Feb 8, 2009, 2:05 PM
Post #8 of 49
(3718 views)
Shortcut
Registered: Oct 20, 2008
Posts: 32
|
irregularpanda wrote: Fuck all that friction shit. Wizards live in the pipe. Kepp them happy with sacrificial goats and do your happy dance.
lol
|
|
|
|
|
RobDude
Feb 8, 2009, 3:18 PM
Post #9 of 49
(3679 views)
Shortcut
Registered: Jan 20, 2009
Posts: 25
|
spoon wrote: Frictional force on the rope is determined in part by the magnitude of the normal force, or how hard it is pushing on the surface against which it is rubbing. Think of it like this. If you try to pull a sled across the ground it's pretty easy. If you try to pull the same sled full of rocks across the same ground it's much harder. When your girlfriend is pulling in slack, there is hardly any weight on the rope so there's hardly any friction, (that's the empty sled). When you are leaning back on the rope, there's lots of weight on the rope so there's a whole lot of friction (that's the sled full of rocks). I hope that helps.
Okay - that actually does help. Thanks a bunch.
|
|
|
|
|
grantjk
Feb 8, 2009, 3:46 PM
Post #10 of 49
(3668 views)
Shortcut
Registered: Mar 14, 2006
Posts: 76
|
I got an A in high school physics, but there's been a lot of years and a lot of hard livin' since then....But, not enough to know that a pulley gives no mechanical advantage. Two people of equal mass on opposite sides of the pulley would hang; add any mass to one side and that side would go down and the other would go up.
What the OP is missing is that when he weights the rope, the force is not great enough to raise his GF's mass, but when she takes in slack, she is only lifting the mass of the unweighted rope. She would not be able to take in rope if he were hanging on it.
There is a point where the difference in mass on the two ends of the rope would result in the lighter side being pulled up, and if we knew what that was we could figure out the coefficient of friction in the system. However, I've overtaxed my brain enough for today, and will have to drink an extra beer tonight to recharge.
|
|
|
|
|
billcoe_
Feb 8, 2009, 4:23 PM
Post #11 of 49
(3637 views)
Shortcut
Registered: Jun 30, 2002
Posts: 4694
|
The physics part to pay attention to is "What goes up, must come down". HOW that exactly happens is up to you, via skill and knowledge....being safe is 95% inside of your brain.
BTW, when leading outside, just tie your girlfriend off to a log or rock to increase the mass. Most gyms have something to simulate this in the form a heavy sack of rocks or a chain bolted to the ground.
|
|
|
|
|
dynosore
Feb 8, 2009, 6:11 PM
Post #13 of 49
(3588 views)
Shortcut
Registered: Jul 29, 2004
Posts: 1768
|
0x2102 wrote: Think of the pipe as a pulley - it effects a mechanical advantage of 2:1, so your 185 lb load feels like 92.5 lbs to the belayer. Add in a little friction and the load of the climber on the belayer is reduced even more.
So if I put a rope over a pipe (or better yet a "frictionless" pulley) and have someone of equal weight hang on the other side they're going to lift me up since they have a 2:1 advantage? Think before speaking.......
|
|
|
|
|
scrapedape
Feb 8, 2009, 6:16 PM
Post #14 of 49
(3581 views)
Shortcut
Registered: Jun 24, 2004
Posts: 2392
|
dynosore wrote: 0x2102 wrote: Think of the pipe as a pulley - it effects a mechanical advantage of 2:1, so your 185 lb load feels like 92.5 lbs to the belayer. Add in a little friction and the load of the climber on the belayer is reduced even more. So if I put a rope over a pipe (or better yet a "frictionless" pulley) and have someone of equal weight hang on the other side they're going to lift me up since they have a 2:1 advantage? Think before speaking.......
No, but you will pull the pipe down.
|
|
|
|
|
scottek67
Feb 8, 2009, 7:50 PM
Post #15 of 49
(3525 views)
Shortcut
Registered: Apr 6, 2008
Posts: 515
|
|
|
|
|
|
hobo_climber
Feb 8, 2009, 7:57 PM
Post #16 of 49
(3511 views)
Shortcut
Registered: Jan 30, 2007
Posts: 75
|
0x2102 wrote: Think of the pipe as a pulley - it effects a mechanical advantage of 2:1, so your 185 lb load feels like 92.5 lbs to the belayer. Add in a little friction and the load of the climber on the belayer is reduced even more.
bzzzzt! wrong. this is 1:1 for a 2:1 you need a traveling pulley on the load itself. the reason for the load 'feeling' reduced is friction, or "the force that holds the universe together". I only weigh 130ish lbs and can quite happily catch my regular partner who is closer to 190lbs...
|
|
|
|
|
shockabuku
Feb 8, 2009, 8:02 PM
Post #17 of 49
(3505 views)
Shortcut
Registered: May 20, 2006
Posts: 4868
|
scottek67 wrote: 0x2102 wrote: Think of the pipe as a pulley - it effects a mechanical advantage of 2:1, so your 185 lb load feels like 92.5 lbs to the belayer. Add in a little friction and the load of the climber on the belayer is reduced even more. do they even teach math in american schools? refer to first line.... this might be a retarded question. there also might be some winning answers!
Speaking of...
you should know how to do this by now.
|
|
|
|
|
0x2102
Feb 8, 2009, 11:25 PM
Post #18 of 49
(3460 views)
Shortcut
Registered: May 17, 2005
Posts: 115
|
adatesman wrote: 0x2102 wrote: Think of the pipe as a pulley - it effects a mechanical advantage of 2:1, so your 185 lb load feels like 92.5 lbs to the belayer. Add in a little friction and the load of the climber on the belayer is reduced even more. Speaking of failing Physics 101.... This is completely wrong. The correct answer is friction of the rope around the pipe. There's some interesting discussion of this in The Lab in Jim Titt's belay device article.
Crap, I hate it when I randomly make stuff up and end up being totally wrong.
|
|
|
|
|
apeman_e
Feb 9, 2009, 12:31 AM
Post #19 of 49
(3422 views)
Shortcut
Registered: Nov 21, 2008
Posts: 212
|
RobDude wrote: The rope goes up to a large pipe, it's wrapped around once, and then back down. In reply to: if the rope was really wrapped around a pipe, which it isnt, there would be so much friction you wouldnt be able to pull in slack. also, the friction created by the pipe can best be mathematically described as E= +/- (%)X(STFU!)
|
|
|
|
|
apeman_e
Feb 9, 2009, 12:32 AM
Post #20 of 49
(3418 views)
Shortcut
Registered: Nov 21, 2008
Posts: 212
|
me no use quotes good
|
|
|
|
|
Valarc
Feb 9, 2009, 1:56 AM
Post #21 of 49
(3381 views)
Shortcut
Registered: Apr 20, 2007
Posts: 1473
|
apeman_e wrote: RobDude wrote: The rope goes up to a large pipe, it's wrapped around once, and then back down. if the rope was really wrapped around a pipe, which it isnt, there would be so much friction you wouldnt be able to pull in slack. also, the friction created by the pipe can best be mathematically described as E= +/- (%)X(STFU!)
You fail at physics too. I've seen several gyms that do a full wrap around the belay bar to intentionally add friction.
You could probably use Curt's foot belay without a second thought in these gyms - it takes very little effort to hold a climber up, which is the whole point. Even if an idiot drops his partner, with a wrapped belay bar, the partner will probably float to the ground slowly enough to limit the injuries.
Personally I think it just encourages lazy, incompetent belaying, but it's certainly done on a somewhat regular basis, and doesn't make it impossible to pull in slack.
|
|
|
|
|
looserock
Feb 9, 2009, 2:33 AM
Post #22 of 49
(3354 views)
Shortcut
Registered: Aug 25, 2008
Posts: 28
|
RobDude wrote: . . . .. . Personally I think it just encourages lazy, incompetent belaying, but it's certainly done on a somewhat regular basis, and doesn't make it impossible to pull in slack.
I think it's safer for kids, even if it teaches bad habits, that just means more attention should be made about those wrapped gym bars vrs regular outdoor top roping. If I owned a gym, I would have those 2" bars double wrapped as well, as they protect a lot of kids. I've seen real new, not even newbe, people come in and climb 30'.; they would drop people without the friction.
(This post was edited by looserock on Feb 9, 2009, 3:20 AM)
|
|
|
|
|
clews
Feb 9, 2009, 5:20 AM
Post #23 of 49
(3304 views)
Shortcut
Registered: Nov 10, 2008
Posts: 190
|
0x2102 wrote: Think of the pipe as a pulley - it effects a mechanical advantage of 2:1, so your 185 lb load feels like 92.5 lbs to the belayer. Add in a little friction and the load of the climber on the belayer is reduced even more.
you're so fuckin dumb it's scary... i really hope you're joking
|
|
|
|
|
majid_sabet
Feb 9, 2009, 5:27 AM
Post #24 of 49
(3297 views)
Shortcut
Registered: Dec 13, 2002
Posts: 8390
|
Valarc wrote: 0x2102 wrote: Think of the pipe as a pulley - it effects a mechanical advantage of 2:1, so your 185 lb load feels like 92.5 lbs to the belayer. Add in a little friction and the load of the climber on the belayer is reduced even more. [image]http://www.facepalm.org/img.php[/image]
I am in love with your wife
|
|
|
|
|
hosh
Feb 9, 2009, 7:41 AM
Post #25 of 49
(3247 views)
Shortcut
Registered: Dec 15, 2003
Posts: 1662
|
majid_sabet wrote: RobDude wrote: So, this is probably a retarded question. But it's been bugging me.... I'm about 180-185 pounds. My girlfriend is probably 125-130. How, exactly, is she able to belay me?!? We climb indoors (so far) and we've always top-roped. The rope goes up to a large pipe, it's wrapped around once, and then back down. There is no pulley system, as far as I understand it, no mechanical advantage....if one end of the rope goes up 2 feet, the other end goes down 2 feet. So, when I get to the top and am being lowered down; why doesn't she get lifted off the ground? Again, obviously she doesn't and all - but I don't get it. My initial thought was that there is a significant amount of friction caused by the rope being wrapped around the pipe. Let's say '100 pounds' worth. So, my 185 - 100 = 85. And since she is > 85 pounds, she'd remain on the ground....but that doesn't make any sense either - because if it took 100 pounds of force to move the rope - she'd be unable to pull in the slack on the rope. So - what gives? (Yes, I'm a 'tard) A biner produces somewhere in the 35 % fraction and that larger tube, I assume takes about 50% so you need to get fatter to lift her up. And by the way, you got an F on your math
And you, my friend, get an "F" for spelling.
hosh.
|
|
|
|
|
|
|
|
|
1
