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climb_eng


Oct 5, 2007, 10:19 AM
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Re: [pastprime] Kilonewtons? a question for those who understand physics [In reply to]
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Pastprime, after reading all of your posts.... lets get one thing clear: I use kgf all the time, it's convenient and easier to imagine the N.

That being said, in technical calculations it's far more convenient to use N rather then kgf or lbf because it allows things to work out dimensionally. If one were to use kgf, you'd have to have a conversion factor (9.81 m/s2) to convert it to kgm.... otherwise many calculations would not work out dimensionally (and therefore be wrong). N are necessary for technical calculations, and make these calculations for simpler then attempting to use kg and as a measure of both mass and force.

AS for climbing gear, you do realize that you can multiply a kN rating by 100 to give you the approximate kgf rating, or 98.1 if you want the accurate kgf rating? Simple easy, no need to rant :).


pastprime


Oct 5, 2007, 11:52 AM
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Re: [rgold] Kilonewtons? a question for those who understand physics [In reply to]
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Thanks rgold and climb eng, for rational replies.

I want to say for the severalth time, I'm quite clear on the principles.

It is my opinion that it helps, not hinders, understanding when, as much as possible, terms are used that the person is already familiar with, which, despite all the howliing, will not give incorrect results. We can explain later that they will weigh less on mars if they ever go there.

I don't think anyone has a problem with the concept that they will weigh less on the moon. I know for sure that a lot of people have trouble relating to kilonewtons.

I now remember when climbing literature gave breaking forces in both pound and Kilograms. No one had trouble understanding what that meant. I know that, because I worked in shops selling climbing equipment, and taught a lot of climbing courses in those times.

Before you blast me for leading my poor clients or customers astray, remind yourself that I am talkiing about a time when this was the same terminology used in the Chouinard catalog, on the hang tags on the ropes, and everywhere else in the climbing world.
No one had trouble understanding it, no one had trouble understanding that they would weigh less on the moon, and no one had accidents atributable to having thought of things in terms of pounds or kg. It worked fine.

When I taught climbing classes, I took it very, very seriously, and I made sure that every single person completely, totally understood everything we covered.
The goal was to get them to really understand the principles so they understood why they were doing what they were doing, and so they could figure things out on their own when confronted with a situation that wasn't in the textbook.

I needed to explain such things as fall factors, and impact force, in a way that gave a clear understanding of what was going on. The clients easily understood forces explained in terms of multiples of body weight, and anyone can make correct desicions regarding climbing procedures by evaluating whatever they are considering in terms of how it will, or won't, multiply the affects of body weight on the gear. I can hear the physics nazis screaming at that statment from here. It doesn't matter. You can make safe, and accurate, climbing decisions thinking that way, and I'm not going to waste the clients money and potential rock time by wasting time explaining what the hell a Kilonewton is, and have that buzzing around in their head, wondering if they understood it correctly, when I have gone on to something else that is really important for them to get.
Anyone who thinks you can't understand climbing pysics without using the word kilonewton is hopeless. I can state with certainty the term was never used by Tom Frost, or Royal Robbins, or Bill Forrest, or the sales rep for Chouinard Equipment, who was a good friend, or the distributor for SMC and Edelrid Ropes, who I worked for, or the product manager for Mammut Ropes, during extensive conversations I have had with them when we were talking about climbing gear, and, to the point, climbing forces. I have made it a point not to drop names on this forum up until now, but I am sick of this bullshit. Incidently, none of the above, some of whom have a well deserved reputation for not suffering fools, in many, many, hours of conversation, ever gave the slightest indication that they didn't think I understood what we were talking about.

When I teach climbing, I want that person to understand what is going on, and the result of various options and decisions. I am not peparing them for a college entrance exam, written and administerd by a bunch of tight assed parrots who can't think in terms other than the way they were taught themselve.

If using the term buttplug helps my students get it, I will use it.


(This post was edited by pastprime on Oct 5, 2007, 12:22 PM)


ptlong


Oct 8, 2007, 1:08 PM
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Re: [glytch] Kilonewtons? a question for those who understand physics [In reply to]
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glytch wrote:
ptlong wrote:
glytch wrote:
...my example of a car moving at a constant speed - is time, in that case, a measure of distance?

If light travels at a constant speed, is time, in that case, a measure of distance?

Uhh, so here's the way this game works. I try to divine the argument you're (not) making, and then debunk it. If I miss the argument you're (again, not) making, well it's not my fault, since you didn't actually make a point.

I assume you're making the connection that a light year is a measure of distance. Well, then, you're right....

Thank you. And by the same token one can speak of the distance of "car hours" without fear of confusion.


pastprime wrote:
I now remember when climbing literature gave breaking forces in both pound and Kilograms. No one had trouble understanding what that meant.

I have a copy of the 1988 Chouinard Catalog. Back then the Camalot (they only sold #1-4) had a strength rating of 1100 kg. The biggest stoppers too. Their carabiners were stamped in kg.

It was understood that kg was a unit of force that converted consistently to pounds or Newtons.


gunkiemike


Oct 9, 2007, 3:08 AM
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Re: [ptlong] Kilonewtons? a question for those who understand physics [In reply to]
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ptlong wrote:

Also worth noting: A kilogram placed on a scale at the North Pole will read differently if placed on the same scale, without recalibration, at the Equator. The difference is small (0.5%), but not insignificant.

Scales also suffer from a buoyancy error. No one seems to acknowledge this. A large 1 gram object (think balloon) will weigh less than a small, dense 1 gram object . Consider - a helium-filled balloon has mass, but no measurable weight (in the lab). The error is potentially much greater than those due to gravity fluctuations.


walkonyourhands


Oct 9, 2007, 3:55 AM
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Re: [Urbain22] Kilonewtons? a question for those who understand physics [In reply to]
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This site's URL should be changed to ROCKPHYSICS.EDU


glytch


Oct 9, 2007, 5:11 AM
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Re: [gunkiemike] Kilonewtons? a question for those who understand physics [In reply to]
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gunkiemike wrote:
ptlong wrote:

Also worth noting: A kilogram placed on a scale at the North Pole will read differently if placed on the same scale, without recalibration, at the Equator. The difference is small (0.5%), but not insignificant.

Scales also suffer from a buoyancy error. No one seems to acknowledge this. A large 1 gram object (think balloon) will weigh less than a small, dense 1 gram object . Consider - a helium-filled balloon has mass, but no measurable weight (in the lab). The error is potentially much greater than those due to gravity fluctuations.

I mentioned just that earlier in the thread - I was asking if a balloon filled with helium had a mass of negative kilograms. Antimatter, anyone?

I really don't understand what the difficulty is with kilonewtons. Sure, companies used to stamp ratings in kilograms - I think I have some ovals with kgs stamped. That said, I think I've adequately flogged the horse of technical correctness, and I won't repeat myself here.

What I don't understand is exactly why it's so difficult to think in kN. If it's a bodyweight-thing, just tell people that 1kN is equivalent to a big dude. Easy. Alternatively, 1kN is 100 kg. Just as easy. Is it really that hard to explain to people? Seriously? As I mentioned earlier, most people have no perspective on how much force a few thousand pounds is. Above 10x bodyweight, I find it hard to believe that most people can honestly discuss factor of 2 differences between forces. 2500lbs or 5000lbs? 12kN or 24kN? the only reason that we have any sense of these things, as climbers, is that we've read about tests, literature, and gear ratings, so we know, for instance, that a first fall on a dynamic rope won't generate more than 10kN, or about 2200lbs of force, or 1000 'kgf'. What does 2200lbs mean to the average person? not very much, really. I see no disadvantage to teaching people technically correct terminology (ie. actual units of force, preferably metric => kN) to begin with.


jt512


Oct 9, 2007, 2:48 PM
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Re: [glytch] Kilonewtons? a question for those who understand physics [In reply to]
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glytch wrote:
gunkiemike wrote:
ptlong wrote:

Also worth noting: A kilogram placed on a scale at the North Pole will read differently if placed on the same scale, without recalibration, at the Equator. The difference is small (0.5%), but not insignificant.

Scales also suffer from a buoyancy error. No one seems to acknowledge this. A large 1 gram object (think balloon) will weigh less than a small, dense 1 gram object . Consider - a helium-filled balloon has mass, but no measurable weight (in the lab). The error is potentially much greater than those due to gravity fluctuations.

I mentioned just that earlier in the thread - I was asking if a balloon filled with helium had a mass of negative kilograms. Antimatter, anyone?

But the balloon doesn't have negative weight either, so your example did not even support the incorrect point you were trying to make.

In reply to:
I really don't understand what the difficulty is with kilonewtons. Sure, companies used to stamp ratings in kilograms - I think I have some ovals with kgs stamped. That said, I think I've adequately flogged the horse of technical correctness, and I won't repeat myself here.

Unbelievable. After your argument has been refuted repeatedly, you still you're "technically correct?"

Jay


glytch


Oct 9, 2007, 3:41 PM
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Re: [jt512] Kilonewtons? a question for those who understand physics [In reply to]
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In reply to:
But the balloon doesn't have negative weight either, so your example did not even support the incorrect point you were trying to make.

Jay, you are one stubborn dude. As rgold, climb_eng, ptlong, gunkiemike, valarc, and others have said quite clearly, if you express a force as a 'kilogram of force', what you're saying is that that force is equal in magnitude to the force that a kilogram exerts on a scale at the earth's surface. You're assuming that a certain mass (in kg) is accelerated at a specific rate (acceleration of gravity at the earth's surface); that gives you the m and the a in F = m*a. You then refer to the force, F by the mass, m, assuming a constant a. What I was pointing out (as ptlong and gunkiemike were noting as well), is that such an assumption can be misleading. ptlong referred to inconsistencies between gravity at the poles and the equator, and gunkiemike to the fact that buoyancy in air can foil naive force measurements as well. The point here is that the notion of a 'kilogram of force' necessarily makes assumptions about acceleration due to gravity at the earth's surface based on the fact that a kilogram is a unit of mass; these assumptions may not be well-defined or as straightforward (and therefore as reliable) as they may appear.

In reply to:
Unbelievable. After your argument has been refuted repeatedly, you still you're "technically correct?"
Jay

Yup, you're right. I'm wrong and nobody agrees with me. Especially not those with well respected opinions in these parts. A sampling:

curt wrote:
And no, a kg can not be used as a unit of force without assuming some things not explicit in the actual definition of a kg.

rgold wrote:
Best to keep the ideas separate, even if the fact that mass and weight are proportional near the surface of the earth allows us to confuse them.


(This post was edited by glytch on Oct 9, 2007, 3:45 PM)


ptlong


Oct 9, 2007, 4:14 PM
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Re: [glytch] Kilonewtons? a question for those who understand physics [In reply to]
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glytch wrote:
...As rgold, climb_eng, ptlong, gunkiemike, valarc, and others have said quite clearly, if you express a force as a 'kilogram of force', what you're saying is that that force is equal in magnitude to the force that a kilogram exerts on a scale at the earth's surface. You're assuming that a certain mass (in kg) is accelerated at a specific rate (acceleration of gravity at the earth's surface); that gives you the m and the a in F = m*a. You then refer to the force, F by the mass, m, assuming a constant a. What I was pointing out (as ptlong and gunkiemike were noting as well), is that such an assumption can be misleading. ptlong referred to inconsistencies between gravity at the poles and the equator, and gunkiemike to the fact that buoyancy in air can foil naive force measurements as well. The point here is that the notion of a 'kilogram of force' necessarily makes assumptions about acceleration due to gravity at the earth's surface based on the fact that a kilogram is a unit of mass; these assumptions may not be well-defined or as straightforward (and therefore as reliable) as they may appear.

Maybe that's what you think I said. Let me be clearer.

When kg is used as a measure of force it is not dependent upon its location on Earth or even being on Earth. It converts consistently to other units of force. There's a constant conversion factor between kg and pounds or Newtons. It's so simple that your objections are silly.


jt512


Oct 9, 2007, 5:54 PM
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Re: [glytch] Kilonewtons? a question for those who understand physics [In reply to]
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glytch wrote:
In reply to:
But the balloon doesn't have negative weight either, so your example did not even support the incorrect point you were trying to make.

Jay, you are one stubborn dude. As rgold, climb_eng, ptlong, gunkiemike, valarc, and others have said quite clearly, if you express a force as a 'kilogram of force', what you're saying is that that force is equal in magnitude to the force that a kilogram exerts on a scale at the earth's surface.

Yes, that is how you are defining a kilogram of force.

In reply to:
You're assuming that a certain mass (in kg) is accelerated at a specific rate (acceleration of gravity at the earth's surface); that gives you the m and the a in F = m*a.

No. When you use kg as a unit of force, you are have not assumed anything. You are just using the unit in accordance with its definition, which refers to a certain acceleration.


In reply to:
What I was pointing out (as ptlong and gunkiemike were noting as well), is that such an assumption can be misleading.

Since there is no assumption, there is nothing to be misleading.

In reply to:
ptlong referred to inconsistencies between gravity at the poles and the equator, and gunkiemike to the fact that buoyancy in air can foil naive force measurements as well. The point here is that the notion of a 'kilogram of force' necessarily makes assumptions about acceleration due to gravity at the earth's surface based on the fact that a kilogram is a unit of mass; these assumptions may not be well-defined or as straightforward (and therefore as reliable) as they may appear.

Maybe if you read this enough times, it will eventually sink in: In defining kg-f, no assumptions about the magnitude of the acceleration due to gravity were made whatsoever. Rather, a magnitude was chosen (the distinction between making a choice and making an assumption is not trivial, and seems to be at least partly the source of your confusion). This magnitude is 9.80665 m/s^2 exactly. Thus 1 kg of force (1 kg-f if you like) is equal to the force needed to accelerate a 1 kg mass 9.80665 m/s^2 exactly. See http://en.wikipedia.org/wiki/Kilogram-force and http://en.wikipedia.org/wiki/G-force.

In reply to:
In reply to:
Unbelievable. After your argument has been refuted repeatedly, you still you're "technically correct?"
Jay

Yup, you're right. I'm wrong and nobody agrees with me. Especially not those with well respected opinions in these parts. A sampling:

curt wrote:
And no, a kg can not be used as a unit of force without assuming some things not explicit in the actual definition of a kg.

rgold wrote:
Best to keep the ideas separate, even if the fact that mass and weight are proportional near the surface of the earth allows us to confuse them.

Curt's statement makes no sense and rgold's doesn't appear to address the question at hand. But if you need appeals to authority to convince you that kg-f is a legitimate unit of force, and pastprime's quotation from his physics textbook aren't sufficient, then spend 5 minutes doing an internet search. You'll find many authoritative sources who state the definition precisely.

Jay


(This post was edited by jt512 on Oct 9, 2007, 6:01 PM)

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