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adatesman
May 26, 2010, 8:41 PM
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adatesman
May 26, 2010, 8:43 PM
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ptlong
May 26, 2010, 8:44 PM
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adatesman wrote: dugl33 wrote: Well, technically 224.81... But if you want to get picky it's 224.8089... 
It is precisely equal to 224.808943 lbs.
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curt
May 26, 2010, 8:55 PM
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ptlong wrote: adatesman wrote: dugl33 wrote: Well, technically 224.81... But if you want to get picky it's 224.8089... It is precisely equal to 224.808943 lbs.
Excellent--and that only took six pages. 
Curt
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redlude97
May 26, 2010, 8:56 PM
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adatesman wrote: Following from that, when combined with the common misconception of pounds and kilograms being similar units, your answer will lead to the incorrect assumption that kilonewtons and kilograms are similar, which is decidedly false. And seeing as the full answer avoids this issue and is easy to understand, why bother adding to the confusion?
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dugl33
May 26, 2010, 8:57 PM
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adatesman wrote: dugl33 wrote: curt wrote: adatesman wrote: jt512 wrote: I'd give it a C–. It's wordy and contains too many irrelevancies. Seriously, the answer to "What is a kilonewton?" is "It's a unit of force in the metric system that is equal to 225 lb." Jay But is it not technically correct? Because it's 224.8? Curt Well, technically 224.81... But if you want to get picky it's 224.8089...
No, if I wanted to get picky I'd be asking you to justify the number of significant figures in your answer.
Personally, I'm happy just doubling the kn number and adding two zeros and calling it roughly that many pounds. If typical loading is in the range 30-50% of capacity I really can't see the need for such accuracy.
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jt512
May 26, 2010, 9:00 PM
Post #133 of 311
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adatesman wrote: jt512 wrote: adatesman wrote: jt512 wrote: adatesman wrote: jt512 wrote: I'd give it a C–. It's wordy and contains too many irrelevancies. Seriously, the answer to "What is a kilonewton?" is "It's a unit of force in the metric system that is equal to 225 lb." Jay But is it not technically correct? As far as I can tell, yes. Jay Then why make the assumption that the full, correct answer is not actually the answer that the OP was looking for and instead provide an arguably incorrect, shorter answer that may have left him even more confused? First of all, I've seen the question asked by climbers on the internet or in person something on the order of 50 times, and every single time all the climber needed and wanted to know was that *sigh* a kilonewton is a unit of force equal to 225 lb. Secondly, that answer is not "arguably incorrect." It's correct, period, at least to three significant figures. Third, to someone who is so scientifically ignorant that they have to ask the question in the first place, it is ridiculous to argue that that one-sentence answer would be more confusing to them than a wordy, convoluted monologue involving a constellation of irrelevant (to them) factoids about time, distance, force, weight, and mass, in two different systems of measurement. Jay Following from that, when combined with the common misconception of pounds and kilograms being similar units, your answer will lead to the incorrect assumption that kilonewtons and kilograms are similar, which is decidedly false. And seeing as the full answer avoids this issue and is easy to understand, why bother adding to the confusion?
You haven't heard a word I've said in this whole thread.
In reply to: (edit- Think I'll pass on the 'correct to 3 significant figures' thing, as that'll be at least another half dozen pages of unnecessary bickering.)
At least that's one matter that you won't be making a fool out of yourself over.
Jay
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hafilax
May 26, 2010, 9:10 PM
Post #134 of 311
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I don't know which is worse: the fact that you guys are so bored that you're arguing this shit or that I'm so bored that I'm reading it.
It's both fascinating and banal at the same time.
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edm
May 26, 2010, 10:01 PM
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redlude97 wrote: When something is weighed in SI units and is given a "mass" in kg, it is actually a weight in N that is then converted based on an assumed constant gravity.
Not necessarily. Spring scales measure force or weight, while balances measure mass. It depends on what instrument you use to do the weighing.
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ptlong
May 26, 2010, 10:01 PM
Post #136 of 311
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rgold wrote: Jay's statement is logically and physically correct. There is no formula involving rope dynamics that involves m as an isolated term, as opposed to mg=w. (This would include falling on slabs, penduluming, pushing off, etc.) It follows that knowing w is sufficient for all possible calculations, and so a knowledge of m is not required if w is available.
I hesitate to contradict rgold, but I can think of a climbing situation where his assertion is not true. It is one which I have embarrassingly experienced myself one time long ago.
It goes like this: The belayer is tethered with the rope loosely to an anchor and is belaying the climber off his waist. The climber falls and pulls the belayer off his stance and the belayer falls until his rope tie-in catches him. During the time that he is falling, the force on him is not his weight alone, but is equal to his weight plus his mass times the tension in the rope.
A picky example in a very picky thread!
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rgold
May 26, 2010, 10:42 PM
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Interesting...but mass times rope tension = mass times force, which doesn't have the correct units to be a component of force.
Aren't the forces on him (before the tie-in comes into play and ignoring air resistance) just the sum of his weight and the rope tension?
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ptlong
May 26, 2010, 11:23 PM
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rgold wrote: Interesting...but mass times rope tension = mass times force, which doesn't have the correct units to be a component of force. Aren't the forces on him (before the tie-in comes into play and ignoring air resistance) just the sum of his weight and the rope tension?
Gaak! I knew I should have hesitated longer.
I should probably quit while I'm behind, but how about a pendulum? The centrifugal force depends on mass and will affect the amount of rope stretch.
Or what if you were charged and in an electric field? Ah nevermind...
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jt512
May 26, 2010, 11:30 PM
Post #139 of 311
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ptlong wrote: rgold wrote: Interesting...but mass times rope tension = mass times force, which doesn't have the correct units to be a component of force. Aren't the forces on him (before the tie-in comes into play and ignoring air resistance) just the sum of his weight and the rope tension? Gaak! I knew I should have hesitated longer. I should probably quit while I'm behind, but how about a pendulum? The centrifugal force depends on mass and will affect the amount of rope stretch. Or what if you were charged and in an electric field? Ah nevermind...
Now we're talking Internets!
Jay
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rgold
May 27, 2010, 12:07 AM
Post #140 of 311
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Yes, you could make a case for a pendulum fall or more generally for any motion that isn't in a straight line, in which case there will be a force of magnitude ma where a is not the gravitational acceleration vector.
So, my statement was as hasty as your first response. But the defect lies with my statement, not with the principle Jay originally articulated.
Mass and weight are equivalent on the earth's surface (practically speaking in terms of climbing computations); all formulas can obviously be written in terms of weight without any reference to mass by simply replacing m by w/g , in which the only variable is w.
I must say I also agree that the referenced article is awful. Surely not what you want to tell a guy who asks what kN on his carabiner means, or at least not the first thing you want to tell him, which would be the conversion to pounds.
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raingod
May 27, 2010, 1:10 AM
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I find the conversion to pounds is one of the last things I would tell someone, as it often leads to the next question. Which is "I only weigh 160 pounds why does my climbing gear need to be rated so high?"
(Also I'm Canadian and have been raised to hate the imperial system)
Ron
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patto
May 27, 2010, 1:28 AM
Post #142 of 311
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kennoyce wrote: I'm really not trying to start a pissing match like Jay and Aric are doing, but you do need to get some facts straight  I also thank you for your quality replies.
kennoyce wrote: Sorry patto, it does look like you are a bit confused on several things.
Both those link CLEARLY support my point of view. In the wiki pound-force article it clearly defines pound-force in terms of a 'pound' (mass). In the slug article the only time 'pound' is unencumbered by a suffix is when it is referring to mass.
kennoyce wrote: Where you go wrong is the reason for this use. A SWL or WLL is still a measurement of force not mass. The reason it is below the breaking strength is not to account for dynamic loading, but to help avoid costly QC testing and lawsuits. Generally the SWL or WLL is determined by finding the breaking strength and dividing it by either 5 or 10 for the factor of safety.
kennoyce wrote: Once again the purpose of this is to be a maximum recommended force seen by the product, not mass. I hope that helps you understand it more clearly.
The figures usually given for retail and industrial products are normally in units of MASS. In the US it could be ambiguous but internationally it is not. All this makes sense because in most cases home and industry users don't know FORCE they know MASS.
kennoyce wrote: What I said above is completely correct as long as the acceleration on the mass doesn't exceed "g" as I stated. In this case the carabiner will be completely safe because the maximum force it can withstand will not be exceeded. Of course here on earth that would mean that the mass would have to be static
(This post was edited by patto on May 27, 2010, 1:31 AM)
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redlude97
May 27, 2010, 1:47 AM
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patto wrote: Both those link CLEARLY support my point of view. In the wiki pound-force article it clearly defines pound-force in terms of a 'pound' (mass). In the slug article the only time 'pound' is unencumbered by a suffix is when it is referring to mass.
wiki wrote: The pound-force or simply pound (abbreviations: lb, lbf, or lbf) is a unit of force.
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acorneau
May 27, 2010, 1:53 AM
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Let me just take a moment to remind everyone that this discussion won't go anywhere unti we all agree that the correct units to use when discussing this sort of thing is PTFTW's....
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curt
May 27, 2010, 1:54 AM
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redlude97 wrote: patto wrote: Both those link CLEARLY support my point of view. In the wiki pound-force article it clearly defines pound-force in terms of a 'pound' (mass). In the slug article the only time 'pound' is unencumbered by a suffix is when it is referring to mass. No, reread this line near the top. wiki wrote: The pound-force or simply pound (abbreviations: lb, lbf, or lbf) is a unit of force. Any time you hear the term "pound" used, it is generally understood to mean lbf. Unless you state "pound-mass" then it is to be assumed to be a force.
That's correct and anyone who claims to have been trained in physics should know that. The generic "pound" unit has always traditionally referred to weight--and not to mass. Unless, of course, you're making reference to the currency, which is a different "pound" altogether.
Curt
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adatesman
May 27, 2010, 2:35 AM
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curt
May 27, 2010, 2:48 AM
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adatesman wrote: curt wrote: redlude97 wrote: patto wrote: Both those link CLEARLY support my point of view. In the wiki pound-force article it clearly defines pound-force in terms of a 'pound' (mass). In the slug article the only time 'pound' is unencumbered by a suffix is when it is referring to mass. No, reread this line near the top. wiki wrote: The pound-force or simply pound (abbreviations: lb, lbf, or lbf) is a unit of force. Any time you hear the term "pound" used, it is generally understood to mean lbf. Unless you state "pound-mass" then it is to be assumed to be a force. That's correct and anyone who claims to have been trained in physics should know that. The generic "pound" unit has always traditionally referred to weight--and not to mass. Unless, of course, you're making reference to the currency, which is a different "pound" altogether. Curt <boredom> Are we back to the weight vs mass thing again? And while we're rehashing things, why hasn't Jay admitted his tape measure is marked in inches rather than decimal pounds and that dealing with inches makes calculating impact force in English units more work than it is in Metric??? </boredom>
Since you have been consistently wrong in this thread, I suppose strawman submissions are about the best you can do.
Curt
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bill413
May 27, 2010, 2:49 AM
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acorneau wrote: Let me just take a moment to remind everyone that this discussion won't go anywhere unti we all agree that the correct units to use when discussing this sort of thing is PTFTW's....
Are those masses or weights?
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adatesman
May 27, 2010, 3:13 AM
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patto
May 27, 2010, 3:19 AM
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redlude97 wrote: patto wrote: Both those link CLEARLY support my point of view. In the wiki pound-force article it clearly defines pound-force in terms of a 'pound' (mass). In the slug article the only time 'pound' is unencumbered by a suffix is when it is referring to mass. No, reread this line near the top. wiki wrote: The pound-force or simply pound (abbreviations: lb, lbf, or lbf) is a unit of force. Any time you hear the term "pound" used, it is generally understood to mean lbf. Unless you state "pound-mass" then it is to be assumed to be a force.
The un suffixed and un bracketted 'pound' could mean numerous things from hitting something, to currency, force or mass. Due to confusing ANY reference needs to be clarified.
However lets look at the definition of a pound-force.
"1 pound-force =={is defined by} 1 pound times standard acceleration due to gravity"
POUND FORCE IS DEFINED by a pound (mass). While there are numerous definitions of a 'pound' the one that has a clear scientific definition is the "The international avoirdupois pound is defined to be exactly 0.45359237 kg" This is what is used in the US.
http://en.wikipedia.org/wiki/Pound_%28mass%29
Furthermore lb is used much more interchangably with kg than lb is with kN. Common usage is more aligned with mass.
curt wrote: That's correct and anyone who claims to have been trained in physics should know that. 
curt wrote: The generic "pound" unit has always traditionally referred to weight--and not to mass. Curt
There is one definition of the pounch which is force. As wiki says:
In reply to: In some contexts, the term "pound" is used almost exclusively to refer to the unit of force and not the unit of mass. In those applications, the preferred unit of mass is the slug, i.e. lbf·s2/ft. In other contexts, the unit "pound" refers to a unit of mass. In circumstances where there may otherwise be ambiguity, the symbols "lbf" and "lbm" and the terms "pounds-force" and "pounds-mass" can be used to distinguish.
But as I mentioned above even this pound force (and slug for that matter) IS DEFINED by pound (mass).
(This post was edited by patto on May 27, 2010, 3:34 AM)
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