# Unit Conversions and Gc? Please resolve this conflict! **URL:** \ **Category:** Factual Questions\ **Created:** [November 30, 2008, 2:21pm UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050 "2008-11-30T14:21:05Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [December 1, 2008, 3:04pm UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/21 "2008-12-01T15:04:09Z")
> [@Derleth](#): > > Only BASIC-derived ones. A wider variety of commonly-used languages (the ones not called BASIC or Basic) use !=. Agree. Perhaps a better way to say this is that the only place I have ever seen “\<\>” is in a programming language (Excel also uses it). ---
**Author:** ![BwanaBob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bwanabob/32/2985_2.png) [@BwanaBob](https://boards.straightdope.com/u/BwanaBob)\ **Post date:** [December 1, 2008, 3:05pm UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/22 "2008-12-01T15:05:43Z")
The fact of the matter is that there is a unit of mass in the Imperial system called the slug. I remember this from physics in high school. From Wikipedia: The slug is an English unit of mass. It is a mass that accelerates by 1 ft/s² when a force of one pound-force (lbf) is exerted on it. Therefore a slug has a mass of 32.17405 pound-mass or 14.5939 kg.[1] Using pounds as mass is pure crap. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [December 1, 2008, 3:13pm UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/23 "2008-12-01T15:13:36Z")
> [@Tabby\_Cat](#): > > 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? As you say, you convert into SI to perform the calculations anyway, and it’s not like the hoi polloi need to know what units you’re working in. Is it really harder to pour concrete in meters? I’m sure China does metric like the rest of the world. So why stay in imperial? Because the U.S. engineering and construction industry (as well as much of the manufacturing industry) are a bunch of hidebound traditionalists. Also, while engineers at least learn the SI system in their general science classes, many technicians and construction workers are not at all familiar with it. This goes back to the general abandonment of converting the U.S. to the metric system. (That is, with the noted exception of the hard sciences, such as chemistry and physics, which are universally taught and worked in using SI units.) In addition, all of the tools and instruments (everything from calipers to survey equipment) are in USCS units. Not to mention standard details, drawings, and specifications. Finally, as I noted above, we don’t use imperial units. We use a system that is derived from the imperial system, but differs in many ways. P.S. Also, I said that _I_ often convert units to SI for easier calculations. This is uncommon, I think. Perhaps because I used to teach chemistry and physics, I’m very familiar and comfortable with the SI system, and appreciate its advantages. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [December 1, 2008, 3:30pm UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/24 "2008-12-01T15:30:30Z")
> [@BwanaBob](#): > > The fact of the matter is that there is a unit of mass in the Imperial system > called the slug. I remember this from physics in high school. > > From Wikipedia: > > The slug is an English unit of mass. It is a mass that accelerates by 1 ft/s² when a force of one pound-force (lbf) is exerted on it. Therefore a slug has a mass of 32.17405 pound-mass or 14.5939 kg.[1] > > Using pounds as mass is pure crap. True enough, but the OP was not about the imperial system. U.S. engineers do not use the imperial system, and I have never seen the slug used by engineers in the U.S. I agree that the unit of pound-mass (lbm) is completely screwed up–but I didn’t invent it, either, so I take no responsibility for it. I do recall the day that the units of pound-mass, pound-force, and _g_[sub]c[/sub] were first introduced in one of my engineering design classes. I remember being absolutely floored by how stupid and confusing it all was. Voicing this to the instructor simply resulted in the instructor noting that this was the industry standard. Finally, as an aside, I’ll note that from a conceptual point of view, while using the pound as a unit of mass is indeed stupid, it’s just as stupid for metric spring scales to be calibrated in units of mass (such as kilograms) instead of newtons. People all over the world routinely confuse force and mass units. ---
**Author:** ![BwanaBob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bwanabob/32/2985_2.png) [@BwanaBob](https://boards.straightdope.com/u/BwanaBob)\ **Post date:** [December 1, 2008, 3:55pm UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/25 "2008-12-01T15:55:04Z")
> [@robby](#): > > …Finally, as an aside, I’ll note that from a conceptual point of view, while using the pound as a unit of mass is indeed stupid, it’s just as stupid for metric spring scales to be calibrated in units of mass (such as kilograms) instead of newtons. People all over the world routinely confuse force and mass units. Truer words were never spoken. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [December 1, 2008, 4:25pm UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/26 "2008-12-01T16:25:46Z")
> [@robby](#): > > True enough, but the OP was not about the imperial system. U.S. engineers do not use the imperial system, and I have never seen the slug used by engineers in the U.S. I have. I was taught to use slugs in most physics and engineering classes, and use slugs preferentially myself. When I taught classes, I taught using both slugs and pounds-mass (as well as SI units). Most of the folks I work with know perfectly well what a slug is, although most also voice a preference for doing calculations in SI units, for reasons that are obvious. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [December 1, 2008, 6:33pm UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/27 "2008-12-01T18:33:09Z")
> [@zut](#): > > I have. I was taught to use slugs in most physics and engineering classes, and use slugs preferentially myself. When I taught classes, I taught using both slugs and pounds-mass (as well as SI units). Most of the folks I work with know perfectly well what a slug is, although most also voice a preference for doing calculations in SI units, for reasons that are obvious. Out of curiosity, when did you graduate? (I graduated from college in the early 1990s.) While I certainly know what a slug is, I have never used the unit. For engineering purposes, I have used only pounds-mass and SI units. When I taught physics, I only used SI units. None of the physics textbooks that I have seen over the last decade have used anything other than SI units. I’ve never heard of the slug as being any more than a historical curiosity. FWIW, the unit is not even mentioned in the supplied reference book for the NCEES Fundamentals of Engineering (FE) exam. ---
**Author:** ![Santo\_Rugger](https://avatars.discourse-cdn.com/v4/letter/s/e95f7d/32.png) [@Santo\_Rugger](https://boards.straightdope.com/u/Santo_Rugger)\ **Post date:** [December 1, 2008, 6:45pm UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/28 "2008-12-01T18:45:01Z")
> [@robby](#): > > True enough, but the OP was not about the imperial system. U.S. engineers do not use the imperial system, and I have never seen the slug used by engineers in the U.S. FWIW, I used slugs extensively in both fluid dynamics and fluid design. I finished my undergrad about three years ago. ---
**Author:** ![robby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robby/32/11048_2.png) [@robby](https://boards.straightdope.com/u/robby)\ **Post date:** [December 1, 2008, 7:33pm UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/29 "2008-12-01T19:33:01Z")
> [@Santo\_Rugger](#): > > FWIW, I used slugs extensively in both fluid dynamics and fluid design. I finished my undergrad about three years ago. I’m quite surprised to hear this. To convince myself that I’m not misremembering things, I just pulled out my copy of _Unit Operations of Chemical Engineering, 4/e_, by McCabe, Smith, and Harriott, published in 1985. This book was the text I used as an undergrad for fluid mechanics (among other topics). Slugs are not even mentioned in the text. There is an introduction to the SI system, as well as what the author calls the FPS (foot-pound-second) Engineering system. In this section, the author discusses the necessity of defining _g_[sub]c[/sub], called the “Newton’s-law proportionality factor for the gravitational force unit.” However, that being said, I’ve also got an introductory book on fluid mechanics (published in 1999) that I picked up in grad school that only uses the SI system, and what the author calls the “British Gravitational” (BG) system of units. The text goes on to state that the book will not use _g_[sub]c[/sub] because it is unnecessary in the SI or BG systems. So I guess the upshot is that it depends on the textbook and the instructors as to whether slugs are still used. However, I’ll note again that the unit is not mentioned in the supplied reference book for the NCEES Fundamentals of Engineering (FE) exam. Only SI units and pounds-mass are used. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [December 1, 2008, 7:50pm UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/30 "2008-12-01T19:50:44Z")
> [@robby](#): > > Out of curiosity, when did you graduate? (I graduated from college in the early 1990s.) Got my undergrad in 1989. > [@robby](#): > > When I taught physics, I only used SI units. None of the physics textbooks that I have seen over the last decade have used anything other than SI units. > > I’ve never heard of the slug as being any more than a historical curiosity. FWIW, the unit is not even mentioned in the supplied reference book for the NCEES Fundamentals of Engineering (FE) exam. Huh. Looking at my own 20-year-old FE (EIT, then) manual, it seems to be nearly exclusively in lbm, reserving slugs for the “English gravitational system.” However, my old dynamics, fluid mechanics, and design books all use slugs preferentially. So, as you say, it probably depends on the preferences of textbook wrriters, and the preferences of the professors who choose the textbooks. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [December 1, 2008, 10:10pm UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/31 "2008-12-01T22:10:50Z")
[QUOTE=Pasta] **Regrading Gc** : What I’ve found online differs slightly from **Chronos** ’s description. ISTM that G[sub]c[/sub] is used more generally for unit conversions, with a different value for different expected input units. Thus F=ma/G[sub]c[/sub], and if you will be working with mass in pounds and acceleration in ft/s[sup]2[/sup] and you want force in N, you’d look up G[sub]c[/sub]=7.22. (It pains me not to give that number units.) [/QUOTE] This is bizarre. I learned g[sub]c[/sub], but not as multiple variations you look up on a table. g[sub]c[/sub] is in units of (ft-lbm)/(lbf-s[sup]2[/sup]). [QUOTE=robby] For instance, in writing Newton’s second law, the equation would be written as _F = ma/gc_, where _F_ is in lbf, _m_ in lbm, and _a_ is in ft/s[sup]2[/sup]. [/QUOTE] I was orginally taught that F=ma needed to have g[sub]c[/sub] added, and then g[sub]c[/sub] for SI was 1. But found that a confusing way to look at it, and unnecessary. g[sub]c[/sub] is just a conversion factor to convert from lbm to lbf. That’s all it is. Like any other units conversion factor, you plug it in when you need to convert between those two sets of units. And yes, all the appropriate units are a necessary part of the conversion factor. [QUOTE=robby] When working out calculations, it is even more critical for U.S. engineers to always write out the units, or you are sure to screw up the calcs. [/QUOTE] I will agree, but think it important to always include the units. That’s one of the double checks you have to ensure you did the math in the right order. Just because the SI units work out neatly without needing an extra conversion factor doesn’t mean you’re any less likely to accidentally invert an operation (flip the numerator for denominator). It also helps keep straight all those powers of 10. Looking up factors on a table (especially ones that don’t have units) seems a sure fire way to miss your errors. ---
**Author:** ![WolfpackJeep](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpackjeep/32/2921_2.png) [@WolfpackJeep](https://boards.straightdope.com/u/WolfpackJeep)\ **Post date:** [December 2, 2008, 12:47am UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/32 "2008-12-02T00:47:05Z")
I’m not familiar with a Gc table; I learned it the way **Irishman** described; specifically, I remember it in the context of “showing all work” when writing out conversions for homework. In my work, I frequently see F=ma carried out where F is just in lbs.; however, there’s often a Gc (no units or other notation, just the “Gc”) written in the equation. I also remember encountering slugs as a unit in homework problems. I graduated in December 2006, in case anyone is wondering. ---
**Author:** ![mnemosyne](https://avatars.discourse-cdn.com/v4/letter/m/c4cdca/32.png) [@mnemosyne](https://boards.straightdope.com/u/mnemosyne)\ **Post date:** [December 2, 2008, 3:32am UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/33 "2008-12-02T03:32:43Z")
This is interesting. I’m an engineering student in Canada, and we are learning both the SI system and what I assume is the food-pound-second system **robby** mentions, with slugs tossed in occasionally when the questions were worded that way to begin with. I have never heard of Gc. All of my profs have been very good about stressing the importance of showing unit conversions, so I guess things just get taken care of by doing the math properly in the first place! ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [December 2, 2008, 3:47am UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/34 "2008-12-02T03:47:18Z")
> [@Derleth](#): > > Only BASIC-derived ones. A wider variety of commonly-used languages (the ones not called BASIC or Basic) use !=. Many dialects of SQL use \<\> as not equals. It’s not just Basic\*. There’s also the ever popular ~= or the old keypunch symbol that looks like a hypen with a tail hanging down from the right end. That symbol followed by = was not equals in PL/I. IIRC there are a few older oddballs (SNOBOL anyone?)which use |= as not equals as well. ---
**Author:** ![Galena](https://avatars.discourse-cdn.com/v4/letter/g/a87d85/32.png) [@Galena](https://boards.straightdope.com/u/Galena)\ **Post date:** [December 2, 2008, 4:50am UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/35 "2008-12-02T04:50:31Z")
I don’t remember much discussion or consternation over foot-pounds, slugs, and the like in my fluid mechanics class. Given, though, that I took it almost 10 years ago, I can’t say I remember much of anything from fluid mechanics. I’m really curious if my students are learning all SI or have to deal with this too. Most of them are currently also taking fluid mechanics. I think I’ll ask around tomorrow, but I have the sneaking suspicion that they’ll look at me like I have three heads. (They do that enough as it is!) Based on how upset they were when I used gallons in class the other day, I’m thinking they actually haven’t had much exposure to engineering in non-SI units. Come on, guys, how else are we gonna do calculations related to the fuel economy of US cars? ---
**Author:** ![Santo\_Rugger](https://avatars.discourse-cdn.com/v4/letter/s/e95f7d/32.png) [@Santo\_Rugger](https://boards.straightdope.com/u/Santo_Rugger)\ **Post date:** [December 2, 2008, 5:27am UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/36 "2008-12-02T05:27:36Z")
> [@robby](#): > > I’m quite surprised to hear this… I just realized it may be due in large part to the fact that my fluid and thermal design instructors used to work in the oil fields. Those petroleum guys mix units as much as explosives guys (cal/g? WTF?). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 2, 2008, 6:21am UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/37 "2008-12-02T06:21:24Z")
What’s so odd about calories per gram? They’re neither of them SI, but they’re both metric, and given that a calorie is defined in terms of the heat capacity of a gram of water, that seems like a perfectly sensible unit to me, in the right context at least. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [December 2, 2008, 6:41am UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/38 "2008-12-02T06:41:09Z")
> [@LSLGuy](#): > > Many dialects of SQL use \<\> as not equals. It’s not just Basic\*. Pascal uses it, too, but there’s a chance they all got it from BASIC: The first version of BASIC was created in 1964, years before both Pascal and SQL. > [@](#): > > There’s also the ever popular ~= This looks more like ‘approximately equal to’ or the Perl-derived ‘regex match’. > [@](#): > > or the old keypunch symbol that looks like a hypen with a tail hanging down from the right end. That symbol followed by = was not equals in PL/I. The symbol is actually the logical NOT operator. It isn’t in ASCII so it wasn’t used in many languages after PL/I’s time. Maybe Unicode will revive it. We _could_ go back to the old FORTRAN conventions and use .NEQ. for this purpose. 😉 ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 2, 2008, 6:56am UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/39 "2008-12-02T06:56:42Z")
Oh, and another one I saw in some programming language or another: /= , which is supposed to look like ≠ . ---
**Author:** ![tim314](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tim314/32/3468_2.png) [@tim314](https://boards.straightdope.com/u/tim314)\ **Post date:** [December 2, 2008, 7:27am UTC](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050/40 "2008-12-02T07:27:31Z")
> [@Irishman](#): > > I was orginally taught that F=ma needed to have g[sub]c[/sub] added, and then g[sub]c[/sub] for SI was 1. But found that a confusing way to look at it, and unnecessary. g[sub]c[/sub] is just a conversion factor to convert from lbm to lbf. That’s all it is. Like any other units conversion factor, you plug it in when you need to convert between those two sets of units. I agree completely. Saying “You always have to add g[sub]c[/sub], but g[sub]c[/sub] = 1 for SI” is ludicrously backwards. Force = mass times acceleration. If instead of telling you the mass, I tell you the object’s weight under standard gravity, then you need g[sub]c[/sub] to convert from weight to mass. [Previous page](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050.md?page=1) [Next page](https://boards.straightdope.com/t/unit-conversions-and-gc-please-resolve-this-conflict/475050.md?page=3)