Forums: Climbing Information: The Lab: Re: [cracklover] Impact force vs. fall length, fixed fall factor: Edit Log




shockabuku


Jul 14, 2011, 7:24 AM

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Registered: May 20, 2006
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Re: [cracklover] Impact force vs. fall length, fixed fall factor
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cracklover wrote:
shockabuku wrote:
cracklover wrote:
shockabuku wrote:
Has anyone seen any data on how the maximum impact force changes (or doesn't change) as fall length increases for a constant fall factor?

Yes, all other things being equal, the longer fall will have a slightly higher peak force.

The reason for that is simple.

Aside from the rope, there are a lot of other things that can "absorb" some energy, such as the sling on the top piece, the climber's body, and most importantly, the knot tightening.

But each of these can absorb a set amount of energy, and unlike the rope, they don't scale with the length of fall.

A small fall will have a smaller amount of kinetic energy that must be "absorbed", and a larger fraction of that will be absorbed by those various other items, leaving a smaller fraction for the rope, and thus a smaller impact force on the top piece. In a large fall, the overall percent of the energy they can convert to heat is much smaller relative to the total amount of kinetic energy, so the rope must absorb essentially all of the energy, putting a higher force on the top piece of gear.

I've only seen one study that looked at how much energy the knot could absorb. Practically speaking, for falls over say ten or fifteen feet, I don't think it made much difference, but if you like I can try to track down that study for you.

GO

You bring up some practical points that I hadn't considered though I'm still interested in the case where those factors are negligible (lab case). I was asking this question because of the thread about Screamers. It linked to some BD research on Screamers that did some test falls of two different fall factors onto different types of slings, screamers, etc. They didn't vary the fall length within the same fall factor though.

Let me put it another way.

The kinetic energy of a falling body is the force of gravity on that object times the distance it falls. IOW, kinetic energy is directly proportional to fall distance.

IOW, you can say that for any object, the energy is equal to the fall distance times a constant.

So, if FF is fall distance / rope, then for a given FF, the rope is always proportional to the fall distance, which is proportional to the energy.

So if you agree that any given section of rope should be able to absorb X amount of energy in the same way that double the amount of rope would absorb double the energy, then, ignoring air resistance and friction and all that jazz, the rope puts the same peak force on your gear for any given FF.

Make sense?

GO

I understand most of what you're saying. I would still like to see some data on a case where these other factors aren't involved.

Strictly speaking you're confusing potential energy (mgh) with kinetic energy (.5mv^2).

This I don't understand "the rope is always proportional to the fall distance, which is proportional to the energy."

This statement

"So if you agree that any given section of rope should be able to absorb X amount of energy in the same way that double the amount of rope would absorb double the energy, then, ..."

seems only to lead to this conclusion

"the rope puts the same peak force on your gear for any given FF."

if the rope is in a linear force vs. distance relationship. I suspect that breaks down with greater fall factors, but I don't know for sure.


(This post was edited by shockabuku on Jul 14, 2011, 7:40 AM)



Edit Log:
Post edited by shockabuku () on Jul 14, 2011, 7:40 AM


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