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pastprime
Oct 4, 2007, 7:34 PM
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I want to isolate this question, because it sums this all up well:
If kilograms are not a measure of force, then how can a scale, which measures nothing but the force being applied to it, be used to count them?
And I reiterate, its not that I don't understand what Kn are, I just think they are dumb.
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rgold
Oct 4, 2007, 7:47 PM
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This probably won't help the confused, but Newton's second law gives the relation between mass and weight, namely F=mg, where g denotes the acceleration due to gravity on whatever planet you're dwelling on.
The point is, for those who stay near the surface of the planet, that g is a constant, and so mass and weight differ by a constant scale factor. This being the case, you can calibrate your scale to report either mass units or weight units.
But, if you want to calculate fall forces in kN for an 80 kg mass climber, you still have to convert that 80 kg mass into an equivalent kN force (by multiplying by the appropriate g-value) in order to obtain correct results from the equations.
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jt512
Oct 4, 2007, 8:03 PM
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pastprime wrote: For those who say kg is not a unit of force, I say it can be used as such, and it works just fine.
Sure, it can be (and is) used as a unit of force, since, as you imply, 1 kg of force can simply be defined as another unit of force times a constant.
In reply to: All definitions of units of force that I know of go back to expressing it in terms of the effects of gravity on a certain mass.
But all of them don't. Specifically, kN doesn't. Its definition derives from the definitions of kg, m, and s, which are in turn defined in terms of the mass (not weight!) of a liter of water, the speed of light, and something about vibrations of a cesium atom, respectively.
In reply to: To those who say weight does not equal force, I ask...
To those who say weight is not a force, I say, go back and take high school physics.
In reply to: A scale, which measures weight, and, on earth, mass...
If a scale "measures" mass on earth, then it must also measure mass wherever else there is gravity. In reality, a scale, however, does not measure mass, only weight; but if you know the value of g, you can calculate its mass from its weight, not only on earth, but anywhere where there is gravity:
F = ma
w = mg, where g is the local acceleration due to gravity
m = w/g.
Thus, wherever g does not equal 0, you can calculate a the mass of an object from its measured weight. But if g = 0, an object has no weight, and hence a scale gives no indication of its mass. Yet it has mass. Therefore, a scale does not measure mass.
Jay
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Valarc
Oct 4, 2007, 8:16 PM
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pastprime wrote: You had me feeling better until you said Kg wasn't a unit of force. If pounds can be a unit of force, so can kg. All you have to do to make it so, is for people to agree that it is.
This argument is idiotic. The kilogram is the SI (that's standards-approved, internationally recognized) unit of mass. Not force. Period. We could all agree to make the buttplug the unit of mass, but that doesn't change the current reality - the kilogram is defined as a unit of mass, and anyone using it for force is being incorrect and sloppy.
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ptlong
Oct 4, 2007, 8:20 PM
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Actually, Jay, the kilogram is defined by a specific chunk of metal kept in France. It is close, but not equal, to the mass of a liter of water. Interestingly, this piece of platinum alloy has been changing ever so slightly when periodically compared to reference copies (change in tens of micrograms). Nobody is sure why, or indeed whether the standard or the references are changing. A transition to a more stable standard is being considered.
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.
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Valarc
Oct 4, 2007, 8:22 PM
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In reply to: If kilograms are not a measure of force, then how can a scale, which measures nothing but the force being applied to it, be used to count them?
A scale DOES NOT MEASURE KILOGRAMS. It measures force, and uses math to convert that value into a value for mass, assuming a certain value for the acceleration due to gravity.
If you want to actually measure mass, you use a balance, which does not directly measure force, but rather compares a mass against other objects of known mass.
Your arguments clearly show that you have no idea what you're talking about - you have a fundamental lack of understanding of the most basic tenets of science. If you take your scale to the top of everest and measure your "weight" in kilograms, it will give a different value than if you used it at sea level, because it's NOT REALLY MEASURING IN KILOGRAMS, it is using an ASSUMPTION to approximate kilograms based on known quantities.
Kilograms are a unit of mass. Period.
You clearly do not understand what kilonewtons are because you're making the same moronic arguments over and over. If you were my student, I'd fail you.
(This post was edited by Valarc on Oct 4, 2007, 8:23 PM)
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glytch
Oct 4, 2007, 8:26 PM
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rgold wrote: This probably won't help the confused, but Newton's second law gives the relation between mass and weight, namely F=mg, where g denotes the acceleration due to gravity on whatever planet you're dwelling on. The point is, for those who stay near the surface of the planet, that g is a constant, and so mass and weight differ by a constant scale factor. This being the case, you can calibrate your scale to report either mass units or weight units.
... exactly what I was saying.
Pastprime, if a car travels at an exact, known speed, is time now the same as distance, since the relationship between time and distance is now completely known? Of course not!
For a weight hanging on an anchor, kilograms may seem like a reasonable choice as a measure of 'force'. However, think of a falling mass being decelerated by a rope. If you say that such a mass is "experiencing a force of x kilograms", what you're actually saying is that the force being exerted by the rope on the mass is the same as the force exerted by a mass of x kilograms hanging on an anchor with the same gravity as is present on the earth's surface. That's a mess.
Seriously, how hard is it to multiply by 9.8 m/s^2? Too hard? Fine, multiply by 10. It's not very hard to go from kilonewtons to kilograms, and back. It's easier than calculating a tip, it's really not all that confusing, and you won't be butchering or misusing any definitions.
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Valarc
Oct 4, 2007, 8:30 PM
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In reply to: Pastprime, if a car travels at an exact, known speed, is time now the same as distance, since the relationship between time and distance is now completely known? Of course not!
Glytch, this was probably the most elegantly worded explanation of this idea I've ever heard. If I ever have the misfortune to have a student arguing this point with me, I'm gonna steal your wording.
Luckily, my students are generally intelligent enough to understand the concept the first time through
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jt512
Oct 4, 2007, 8:57 PM
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Valarc wrote: In reply to: If kilograms are not a measure of force, then how can a scale, which measures nothing but the force being applied to it, be used to count them? A scale DOES NOT MEASURE KILOGRAMS. It measures force...?
Really? Have you ever seen a European bathroom scale? The one's I've seen are scaled in kg, not daN. You need to rethink your position. The kg most certainly can be (and routinely is) used as a unit of force, and pastprime has provided a rigorous definition of the kg as a unit of force.
Jay
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Valarc
Oct 4, 2007, 9:25 PM
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jt512 wrote: Valarc wrote: In reply to: If kilograms are not a measure of force, then how can a scale, which measures nothing but the force being applied to it, be used to count them? A scale DOES NOT MEASURE KILOGRAMS. It measures force...? Really? Have you ever seen a European bathroom scale? The one's I've seen are scaled in kg, not daN. You need to rethink your position. The kg most certainly can be (and routinely is) used as a unit of force, and pastprime has provided a rigorous definition of the kg as a unit of force. Jay
A bathroom scale measures force. However, it uses an assumption about the current acceleration due to gravity to give an approximate value for mass. Kilograms are not being used as a measure of force, but rather the manufacturers of the scale are being sloppy with terminology because they know the intended usage of the device. The device is measuring force but giving an output in mass because that's what people care about. The fixed conversion factor works just fine because the gravitational field of the earth doesn't vary very much. You don't jump up and down on your scale and say LOOK IM EXERTING TWO HUNDRED KILOGRAMS OF FORCE. It's a completely moronic statement and no one would make it. If you submitted a funding request to NASA saying you wanted to measure how many kilograms of force are exerted by the surface tension of a gel you want to test in the space shuttle, you would be laughed out of the scientific community, because kilograms ARE NOT A UNIT OF FORCE.
Sure, you can provide a rigorous definition of force in terms of any weird combinations of units and conversion factors you want, but it doesn't make that definition any more accepted or correct.
Look at it this way - in a car, you drive at 50mph. The speedometer is not really measuring velocity, it is measuring angular velocity and using assumptions about the wheels to give you a value for the speed. You wouldn't claim that miles per hour is a unit of angular velocity, because while that's what the device is measuring, the device is using assumptions to convert the measurement into a more immediately accessible concept.
As far as needing to re-think my position - i will not. I teach this stuff for a living, and I'm not about to get sloppy with my terminology just because some people on the internet told me to.
(This post was edited by Valarc on Oct 4, 2007, 9:30 PM)
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glytch
Oct 4, 2007, 10:33 PM
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Jay,
I don't get it. You _must_ be intelligent enough and sufficiently well educated to recognize that
a) A gram is, definitionally, a unit of mass. If you don't believe this one, well, consult a high school physics text.
and
b) Using the known acceleration by gravity in a vacuum at the earth's surface, the mass of an object can be ascertained by 'weighing' the object, since the object's mass is accelerated in a known manner by gravity.
Note that in part (b), I deliberately included "In a vacuum"; let's get rid of that phrase temporarily. Attach a helium balloon to a scale and read the scale. You'll find that the balloon has a mass of negative kg... uhhh, what? By part (a), you acknowledge that kilograms are a measure of mass. So, the balloon has negative mass? Is it antimatter? What has gone wrong?
Simple: The assumptions that your scale uses to approximate the mass of an object based on its weight have failed. Namely, in the case of a helium balloon in air, the assumption that the balloon is in a vacuum is clearly violated, and the scale returns an erroneous result.
You didn't respond to my example of a car moving at a constant speed - is time, in that case, a measure of distance?
G
edit: typo
(This post was edited by glytch on Oct 4, 2007, 11:31 PM)
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pastprime
Oct 4, 2007, 11:03 PM
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Quote:
"As far as needing to re-think my position - i will not. I teach this stuff for a living."
Thank you. You have made my point with exquisite perfection.
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jt512
Oct 4, 2007, 11:09 PM
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Valarc wrote: As far as needing to re-think my position - i will not. I teach this stuff for a living...
Well that's a little scary, but I strongly suspect that you're teaching at the high school, and not the college level, so no real harm.
Jay
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ptlong
Oct 4, 2007, 11:46 PM
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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?
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jt512
Oct 5, 2007, 12:11 AM
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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?
Beat me to the punch.
Jay
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pastprime
Oct 5, 2007, 1:07 AM
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What you people are failing to grasp is that I understand your side of this completely.
I got A's in my college physics courses by using Kn, Kg, slugs, and all the other terminology exactly the way you want it to be used to show I understood it. I also worked out all the problems in the exercises, both the ones assigned and the unassigned ones, because I thought they were fun; correctly.
I even would have gotten and A from you, Valarc, because, in class, I'd have parroted back the terminology and your explainations of the principles just the way you wanted them, and I would have picked up in the first few days that it would be wise to keep my mouth shut so as not to confuse you with unfamiliar concepts, or get you on your high horse.
What completely gets my head spinning, even after all these years on this planet of the apes, is that someone can seriously say that 2.2 pounds is a legitimate measure of force; no problem with that; but that only a moron would call a kilogram a unit of force; and not, when it is stated that clearly, see that that is nuts.
I am very clear that you have been taught all of your career that kg could not be used as a measure of force. I get that. It is even weirder that you say pounds can be. What I am utterly unable to grasp is how you are completely unable to grasp that just because something is accepted, and published in textbooks, does not mean it isn't silly.
I know you think I'm nuts, or dumb. I get that.
I also clearly get that everything I've brought up in my posts has gone right by you- with the singular exception of Jay.
All you have been able to grasp from what I've said is that i don't go along with what you have been told all your life; and I must- have to be, or your entire world comes to question- be wrong. I just have to be. No point in paying any attention to what I am actually saying, I just have to be wrong, because if you were told it in school by people a lot smarter than you, then it has to be true.
I doubt I'll post on this anymore, because at this point, those who get it, get it, and those who don't, don't, but I stand by my position.
Just please, please, don't say anymore that I don't comprehend your side of it. I can say it your way, and explain it your way, quite clearly. This is a discussion, not a class . Anyone who is unable to grasp that there is something to the concept that a spring scale is incapable of registering anything except the amount of force being applied to it, and that no matter what letters you paint beside the number on its face, it is reacting to the same thing, is beyond hope. Call it pounds, and it is ok to say it is a measure of the downward force being applied. Call it kg, and it is not. That. Is. Nuts. If a scale soes not measure force, then what is happening to it if you have a weight on it and suddenly accellerate it upward, when the scale, according to you, says the mass has just increased? It is measuring force, folks. That is all it knows how to measure.
I'm reminded of what one of the most brilliant teachers I had, said once, though he was quoting someone else. Something to the effect that "If you can't explain the same thing in several completely different ways, using completely different terms, then you don't really understand it."
thanks one and all.
(This post was edited by pastprime on Oct 5, 2007, 1:15 AM)
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hiyapokey
Oct 5, 2007, 1:50 AM
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I thought that it had to be foot pounds to be a measure of force.
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glytch
Oct 5, 2007, 4:13 AM
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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. Congratulations. I now send you to a page of definitions of "light year", courtesy of google:
http://www.google.com/...inition&ct=title
Notice that by every definition listed (which is good, since it's a pretty easy thing to define), a light year is defined as the distance which light travels in a year (implicitly, light is referred to as being in a vacuum. Otherwise a light year isn't well defined, since light does not travel at the same speed through different media). When referring to space, a 'year' is not a unit of distance. A 'light year' is a unit of distance, just as you could take a car that moves at a constant speed and define a 'car-hour' as a unit of distance. A year is not a unit of distance in the case of a light-year, and an hour is not a unit of distance in case of a car-hour.
I believe that addresses the message you were intending to bring up. If you meant something deeper and relativistic, well, let's not go there. I mean, I can, if you want, but then we'll just be talking about the relationships between space, time, and velocity; space and time STILL won't be the same thing, and a year will never be a mile. Sorry.
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glytch
Oct 5, 2007, 4:33 AM
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jt512 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? Beat me to the punch. Jay
Jay, this is the second recent thread in which you have steadfastly ignored arguments that I have made which directly or indirectly went against what you had written. Do you ever actually respond to arguments made against your points?
Maybe I'm just a moronic noob and the things that I write are just so inane that you can't be bothered to respond. In that case, it really shouldn't take you more than a minute or two to craft a witty, intelligent response debunking the crapola that I've been writing.
The way I see it, you take every opportunity to pounce on any person or argument you think you can belittle or ridicule. Maybe you'll write one of those nasty posts responding to me one of these days.... but I'm not holding my breath. See, you're adhering to the old adage: "It is better for people to believe that you are a fool than to open your mouth and prove them right."
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curt
Oct 5, 2007, 4:46 AM
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Valarc wrote: pastprime wrote: You had me feeling better until you said Kg wasn't a unit of force. If pounds can be a unit of force, so can kg. All you have to do to make it so, is for people to agree that it is. This argument is idiotic. The kilogram is the SI (that's standards-approved, internationally recognized) unit of mass. Not force. Period. We could all agree to make the buttplug the unit of mass, but that doesn't change the current reality - the kilogram is defined as a unit of mass, and anyone using it for force is being incorrect and sloppy.
OK, somebody please re-title this thread to:
Buttplugs? a question for those who understand physics. The topic has basically gone to shit anyway. 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.
Curt
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glytch
Oct 5, 2007, 5:03 AM
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pastprime wrote: Call it pounds, and it is ok to say it is a measure of the downward force being applied. Call it kg, and it is not. That. Is. Nuts. If a scale soes not measure force, then what is happening to it if you have a weight on it and suddenly accellerate it upward, when the scale, according to you, says the mass has just increased? It is measuring force, folks. That is all it knows how to measure.
2 points:
1) A little heard-of man named Newton once had some cockamamey idea which has since gained a wee bit of traction in the physics community. That idea was F = m*a. Fascinating, that. Before we begin discussing the implications of F = m*a such as conservation of momentum, or of energy (in a closed system), there's a much more direct and powerful conclusion: There is a difference between force and mass. They are necessarily different, and Newton gave us a relationship between them.
2) A kilogram was not sent down from the heavens, and examined by physicists. A kilogram, believe it or not, is a COMPLETELY fabricated quantity, which represents the mass of matter. There's nothing more or less complicated about it than that. It doesn't take a theoretical physicist to explain such a concept or write about it in a textbook - a kilogram is, by the people who defined the damn thing, a unit of mass. God did not hand us a kilogram, and say "Interpret this" - some dude got a jug o' water that was about a liter and said: "This much mass shall heretofor be known as a kilogram." No opening of one's mind is necessary, no alternate interpretations allowed. A kilogram represents math, by definition. Plain and simple.
I believe that that adequately addresses your condescending "Your physics teacher told you a kilogram was a mass, and you're not creative enough to realize it COULD be a force, too!" argument. Masses and forces are different, and a kilogram is defined as a unit of mass, not force. Fin.
To address your other, less condescending point: Why is it that we can't just rate gear in kilograms - all of the gear is on planet earth, right? Fine, fair question. Here's why: climbing gear is built and/or analyzed by engineers. Preferably mechanical ones. In analyzing the gear, it is necessary for computations to be done which respect the fact that a kilogram is a freakin' mass! The engineers have to treat a kilogram as such, or calculations are blatantly nonsensical. So, the question is now: Why can't you just stamp a kilogram rating on the carabiner, for the layman to understand? Well, first, because lots of climbers are sufficiently well educated that we recognize that there is a (necessary!) difference between a mass and a force. Mainly, though, you would just be misusing technical terms in life-critical applications. No, it might not cause widespread disaster, but it would be blatant misuse of well-defined terms in what is really an engineering application. If you don't have the capacity to understand that there is a fundamental difference between mass and force and you can't adjust from kN to kg by dividing by ~10, you shouldn't be climbing. A company making engineering products should use the correct technical terms in the specifications of their products. They should not dumb-down and label their products with quantities of improper units simply because such a mistake is common in everyday life. If BD labeled their gear in kg, I wouldn't be buying any more hotwires. An engineering firm should know and apply the correct terminology.
Finally, you never addressed my example: What is the 'mass' of a helium balloon if it 'weighs' negative kilograms?
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jt512
Oct 5, 2007, 5:07 AM
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glytch wrote: jt512 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? Beat me to the punch. Jay Jay, this is the second recent thread in which you have steadfastly ignored arguments that I have made which directly or indirectly went against what you had written. Do you ever actually respond to arguments made against your points?
Well, yeah, I do. But you seem to have been attempting to rebut points that I wasn't making.
In reply to: Maybe I'm just a moronic noob and the things that I write are just so inane that you can't be bothered to respond.
There's an element of truth to that, along with the facts that I've been very busy today and that ptlong posted essentially the same response that I was going to when I got out of the shower (I have your permission to occasionally take a shower, I assume).
In reply to: In that case, it really shouldn't take you more than a minute or two to craft a witty, intelligent response debunking the crapola that I've been writing.
Well, here is literally what I was going to post:
"What you are claiming, then, is that no equality relationship can be established between time and distance. Well, if you're right, then you can rightly say that my thinking is light-years out of date."
Personally, I think my response is wittier than ptlong's; but he "published" first, so what can I say.
In reply to: The way I see it, you take every opportunity to pounce on any person or argument you think you can belittle or ridicule. Maybe you'll write one of those nasty posts responding to me one of these days.... but I'm not holding my breath.
Ok, since you insist, how do you like this one: "I'm guessing that you're in Pennsylvania for the climbing, rather than for the University of Pennsylvania." Pretty good, huh?
In reply to: See, you're adhering to the old adage: "It is better for people to believe that you are a fool than to open your mouth and prove them right."
Wiser words were never spoken.
Jay
(This post was edited by jt512 on Oct 5, 2007, 5:10 AM)
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glytch
Oct 5, 2007, 5:22 AM
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jt512 wrote: There's an element of truth to that, along with the facts that I've been very busy today and that ptlong posted essentially the same response that I was going to when I got out of the shower (I have your permission to occasionally take a shower, I assume).
absolutely not.
In reply to: Well, here is literally what I was going to post: "What you are claiming, then, is that no equality relationship can be established between time and distance. Well, if you're right, then you can rightly say that my thinking is light-years out of date." Personally, I think my response is wittier than ptlong's; but he "published" first, so what can I say.
In reply to: Ok, since you insist, how do you like this one: "I'm guessing that you're in Pennsylvania for the climbing, rather than for the University of Pennsylvania." Pretty good, huh?
... not really. Actually, that was sort of lackluster. You can do better than that.
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pastprime
Oct 5, 2007, 4:13 PM
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Dug out one of my old physics texts last night.
Pertinent quote:
"...the approved force unit is now the newton.." (I never questioned that). "The older unit, 'kilogram of force', in no longer use in scientific and technical work. Only a few years ago,.... the kilogram was used both as a unit of mass and as a unit of force, or weight. ..... . In this book, when we use the word kilogram (kg), we will mean mass, not force. If, in some cases, a kilogram of force is to be indicated, we will use the designations 'kilogram of force, or kg-f"
Quite apparently, kg was used as a unit of force, presumably by people who were not idiots, or did not understand physics. In fact, looking at the time frame, the first man on the moon was probably put there using equations with kg as a unit of force.
Using kg, or weight, to give someone an idea of the magnitude of certain forces, is completely legitimate.
It is only current convention that uses the more complicated, and less quickly grasped, kn.
Using kn as a unit of force worked just fine once, and works just fine now, which was my entire point.
If you don't want to use it, and get satisfaction from thinking someone who does is an idiot, fine. Can't argue with either. Your call.
Just quit saying it doesn't, or can't, work, or that someone who uses those terms in their explaination can't possibly understand the principles.
It is interesting how some of those arguing against me, have used some of the very same arguments I used myself in earlier posts. No one (except Jay) seems to have paid much attention to what I was actually saying. They just detected heresy, and plugged in what they thought was an appropriate tape to play back.
(This post was edited by pastprime on Oct 5, 2007, 4:19 PM)
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rgold
Oct 5, 2007, 4:38 PM
Post #75 of 85
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Registered: Dec 3, 2002
Posts: 1804
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Suppose you attach a stone to a piece of string and whirl it around at constant angular velocity. The amount of force required (eg the tension in the string) is determined by the mass of the object and will be the same on the earth, on Mars, and in deep space. If you were to replace mass by weight, you would get three different answers in the above three examples, including an answer of zero in the case of deep space, where the stone has no weight.
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.
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