I’ve never, ever used calories as a unit for anything in a calculation other than when predicting detonation energy. Pretty much all the intermediate calculations are done in calories, which are then compared to TNT (which is defined as having 1,000 calories per gram) to give the equivalence. It seems so odd to me because I’ve never used it for anything else.
>The crux of the misunderstanding is that the fundamental units of SI are mass, length, and time; the fundamental units of the imperial system are force, length, and time.
I don’t think this is the crux. Which units are fundamental and which ones are derived are of interest in the field of metrology, but people applying the technology of measurement don’t need to know.
>Suddenly, the whole “Mars lander crashed because we were using different units” makes more sense.
>Which brings me to my question… why on earth are you still using imperial for engineering?
Oh, God, yes, this is a nightmare. The reason we still use imperial or English or US Customary units is that most of the engineering components used in industry are sold in these sizes, and many physical value references are in these units, and most measurement devices measure with these units. The USA spends an enormous, enormous sum every year carrying all this horrible baggage along, for what seems to those of us doing the carrying absolutely no reason at all.
Almost every single thing I figure out, my first step is betting on whether it will be more efficient to convert to SI at the start and then back at the end, or more efficient to try to use the imperial units without making any mistakes.
And, if you think it’s awkward when calculating forces, just wait till you try heat transfer coefficients!
I disagree. Note that basically the entire discussion has been the difference between pound mass, pound force, the use of slugs, etc. Since mass is not a fundamental unit in the imperial system, you must do some conversion that’s not done in the SI system. We can’t easily measure kilograms directly, but if we measure Newtons it’s child’s play to convert it to kilos. In the customary system, it’s much more confusing (this thread is my cite).
If the reason I gave isn’t the crux, then what is?
It’s the crux in so far as the familiar, commonly used unit is force instead of mass. You could treat the foot, slug, and second as the fundamental units: There’s nothing about the unit system which forces you to call one thing or another “fundamental”. Compare the SI electrical units, where officially it’s the amp that’s fundamental and a coulomb is one amp-second, but where in practice everyone treats it the other way around.
The problem, though, is that slugs are so rarely used, while pounds are ubitquitous. In SI, a quantity of something is typically expressed in kilograms, mass units, while in the American system, such a quantity is typically expressed in pounds, force units.
It’s even further complicated by the fact that there’s at least three different treatments of pounds in the US system(s). You can treat pounds as the force unit and slugs as the mass unit, or you can use pounds as the mass unit and poundals as the force unit, or you can use pounds as the mass and force unit, and throw Gc willy-nilly into all the formulae.
In high school (including advanced physics), I was taught there are the SI and English units. We used slugs all the time for English. At some point, I recall a discussion on how can a scale meaure weight in kg, and not Newtons? This really confused me. This led into the concept of kg-mass (kgm) and its evil twin, the kg-force (kgf). Luckily, knowing 1 kgf = 1kgm makes it simple to side-step the whole issue. It’s like it was dummied-down for the public who couldn’t be bothered to understand Newtons. This discussion led into a mention of pound-mass (lbm) and pound-force (lbm)…which I just accepted as being analogous to the kgm and kgf.
I never heard of the USCS set of units in high school. In college, I recall a mention of USCS units in a drafting class (c.1985-just before CAD came out full-force), but it didn’t mean much. It sure looked like English units to me! Someone should have stressed the subtle differences, but no one did. As I recall, it was in Thermo where we had to solve problems in both SI and English and this started to become problematic. Here, we were taught about “gc” in a sloppy way perhaps by those who lacked a firm understanding themselves! Also, our English units problems were always in lbf and lbm. No mention of “slugs”. The book was even deemed a SI-English version (as it could come as one or the other or both).
Last, in all my years (other than that drafting class), no one ever bothered to differentiate between English and USCS…and I always wondered why we never spoke of “slugs” anymore. Personally, I thought the whole lbm and lbf business was because someone was bothered by calling a unit of measure a “slug”!
It’s not just an academic thing. I use slugs routinely in real-world aerodynamic calculations. If I’m given a density, I expect that the units will be given in slugs/ft[sup]3[/sup]. That is the given unit in most of the US-unit standard atmosphere charts I’ve ever seen. The gas constant for air is far more often given as 1716 ft-lb/slug-°R than as 53.35 lb-ft/lbm-°R. There are definitely advantages to using slugs over lbm, not the least of which is that when you use slugs, you don’t have to use that godawful g[sub]c[/sub]. Now of course we’re just trying to be difficult when we want to see air flow rates in lbm/s. But I don’t make the rules.
Don’t get me wrong…I prefer SI units in principle. I find them easier to work with and less prone to causing me to make mistakes. But I use British units more because they’re customary in the fluid dynamics world. Like it or not, airplanes fly at so-many-thousand feet, weigh so-many pounds, and their engines provide so-many pounds of thrust. When all the numbers you’re given are in Imperial units, it only makes sense to stick with that system. Otherwise you spend all your time converting units back and forth and end up crashing your space probe.
Even worse is that the CFD weenies I have to interact with love to use “snails”, which is equal to 12 slugs (that way, 1 lb = 1 snail * 1 in/s[sup]2[/sup]). Next time they give me a mass flow rate in snails/s, I’ll be sure to slug them. Or pound them.
I wonder if we’re just arguing semantics now. Is it familiar because it’s fundamental, or is it fundamental because it’s familiar? Chicken, egg?
Wiki said they changed it in 1960. Did they change it because it became familiar?
>I disagree. Note that basically the entire discussion has been the difference between pound mass, pound force, the use of slugs, etc. Since mass is not a fundamental unit in the imperial system, you must do some conversion that’s not done in the SI system.
The crux of this discussion is the use of force versus mass, and the further confusion of creating new units that represent the masses associated with various forces or the forces associated with various masses. You can measure mass and force in the Imperial and SI systems. If you want not to use slugs, then there is an inconsistency in the Imperial system to keep track of.
However, the issue of which units are fundamental in any system is an entirely different thing, and it is not of interest or use to people wrestling with the problems in this thread. The organizations like NIST that take responsibility for defining and improving measurement standards have a difficult job reconciling the results of various fascinating experiments in a way that is forward-thinking and yet deals effectively with history too. A year or two ago there was a fascinating article in Scientific American about an attempt to make very precise spheres of crystalline silicon, so that the number of atoms in the sphere could be very accurately calculated (maybe to 7 digits, IIRC). These spheres would then become mass standards, replacing the archaic platinum-iridium alloy master cylinder stored in France, and the several copies of it that normally live in other countries. Those cylinders have to be painstakingly moved together and compared every so often. Worse, the master appears to be gaining weight relative to all the copies. This is the issue of which units are fundamental. This is what makes the issue interesting and important.
There was a fascinating review article in Physics Today perhaps 5 years ago or so about the latest regression analysis of various experimental results relating the various fundamental and derived units. You can also get some idea of this topic browsing the NIST website.
But, in discussions like the OP, it is enough to go looking up (on the NIST website if you are obsessed or in the CRC handbook or anyplace else if you aren’t) the conversion factors between units. You never have to know which ones were the fundamental ones.