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Re: [jt512] Pendulum fall speed:
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skiclimb
Sep 2, 2010, 5:12 AM
Views: 8270
Registered: Jan 11, 2004
Posts: 1938
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jt512 wrote: skiclimb wrote: jt512 wrote: ptlong2 wrote: Like this? [img]http://img825.imageshack.us/img825/3360/horizontala.jpg[/img] About how much slack would you add? I don't know. My gut reaction would be five feet, maybe six. In addition, I'm going to dynamically belay, which I think will also reduce the swing into the wall. Edit: Actually, as drawn, it looks like I can't dynamically belay. Jay In both cases with pro or with short pro adding slack will increase the speed and therefore impact force ..unless maybe you add the full ropelength of slack in the with pro diagram.. I doubt it. At least not in the case with pro. Your reasoning does not take into account that as you increase the slack you decrease the angle at which the pendulum starts. By your own admission, there must be an amount of slack that will reduce the impact force into the wall, but I doubt that it really is a whole rope length. My guess is that the impact force against the wall decreases monotonically with the amount of slack. I assume that ptlong2 will eventually run the numbers and give us his solution. I just had a vague recollection of a similar problem coming up on rec.climbing a long time ago. My memory might be playing tricks on me, but I seem to recall someone claiming that the amount of slack should be greater than the horizontal runout. Jay
If i remeber my old physic correctly a pendulem only redirects force it does not mitigate it.
the amount of vertical fall is the sole generater of speed. more slack means more speed = more force at impact.
the reason i meantioned that a full ropelength fall might be safer (but probably not) is that other forces can come into play ..such as glide and air friction may allow a person to hit the rope at a point directly under the pro without hitting the wall first.
then again it could make it worse if the person falls in such a way that they push away from the wall..
they key is the direction of fall in regarding slack..if they fall toward the wall such that they aproach nearly directly below the pro point once the slack runs out then they have a chance to not hit the wall at all.
However if they fall straight down or only slightly towards the pro or worst of all away from the pro then the slack just increases the speed at which they impact the wall.
best case would be an aproximately 30 foot freefall towards and slightly past the pro with moderate slack such that you hit the rope at a point between the wall and the pro perhaps about 1ft past the pro on the wall side.. the resulting pendo would actually take you away from the wall...
However realworld best bet in this case is a tight belay and the old rule.."the leader must not fall" If the leader must fall he should try to use the last couple second or so of control before failure to jump/pushoff towards the pro
(This post was edited by skiclimb on Sep 2, 2010, 5:21 AM)
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Edit Log:
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Post edited by skiclimb
() on Sep 2, 2010, 5:14 AM
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Post edited by skiclimb
() on Sep 2, 2010, 5:16 AM
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Post edited by skiclimb
() on Sep 2, 2010, 5:18 AM
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Post edited by skiclimb
() on Sep 2, 2010, 5:21 AM
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Post edited by skiclimb
() on Sep 2, 2010, 5:21 AM
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