# Mass & Weight Units **URL:** \ **Category:** Factual Questions\ **Created:** [January 14, 2000, 4:52pm UTC](https://boards.straightdope.com/t/mass-weight-units/6266 "2000-01-14T16:52:00Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [January 14, 2000, 4:52pm UTC](https://boards.straightdope.com/t/mass-weight-units/6266/1 "2000-01-14T16:52:00Z")
The kilogram is a unit of mass, yet most metric scales measure things in kilograms. Shouldn’t a metric scale use Newton units? * * * It’s not a cookie, mum! It’s a fig newton! ---
**Author:** ![Keeves](https://avatars.discourse-cdn.com/v4/letter/k/f05b48/32.png) [@Keeves](https://boards.straightdope.com/u/Keeves)\ **Post date:** [January 14, 2000, 5:00pm UTC](https://boards.straightdope.com/t/mass-weight-units/6266/2 "2000-01-14T17:00:00Z")
On the earth’s surface, for a non-accelerating object, newtons and kilograms are so close to the same thing that anyone who cares about the difference is, in fact, already using the one he cares about. ---
**Author:** ![Keeves](https://avatars.discourse-cdn.com/v4/letter/k/f05b48/32.png) [@Keeves](https://boards.straightdope.com/u/Keeves)\ **Post date:** [January 14, 2000, 5:03pm UTC](https://boards.straightdope.com/t/mass-weight-units/6266/3 "2000-01-14T17:03:00Z")
PS, I had to read the OP several times until I understood the question. You should have phrased itL > [@](#): > > The kilogram is a unit of mass, yet most metric scales measure **weight** in kilograms. Shouldn’t a metric scale use Newtons? ---
**Author:** ![Strainger](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/strainger/32/244_2.png) [@Strainger](https://boards.straightdope.com/u/Strainger)\ **Post date:** [January 14, 2000, 5:03pm UTC](https://boards.straightdope.com/t/mass-weight-units/6266/4 "2000-01-14T17:03:00Z")
Brace, yourself, Jinx. Many times, engineers will express mass and weight units in sort of a bass-ackwards fashion for convenience. For example, a kilogram-force (kg[SUB]f[/SUB]) is the amount of force that a kilogram mass exerts in earth’s gravity (at sea level; let’s not get semantic here). Likewise, engineers will also use units like pound-mass (lb[SUB]m[/SUB]) which is the mass of an objective that exerts one pound-force (lb[SUB]f[/SUB]) in earth’s gravity at sea level. The pound-mass expression was particulary convenient to me when I worked in the aerospace industry, since acceleration and vibration requirements were always expressed in g’s. That saved me from having to multiply g-levels by 32.17 ft/s[SUP]2[/SUP]. ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [January 14, 2000, 5:09pm UTC](https://boards.straightdope.com/t/mass-weight-units/6266/5 "2000-01-14T17:09:00Z")
Strainger - Yes! That’s what I’m getting at! technically, a “slug” should be unit of mass for the English system, but perhaps it is more convenient to talk about pound masses with units of lb-m. So, when something is measured in kg-f, does 1 kg-f = 1 N? Also, one problem with this system is that the units will not work out quite right in some calcs., so engineers will use a “g-c” term to fudge it, right? (The dash is used to indicate a subscript.) ---
**Author:** ![Strainger](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/strainger/32/244_2.png) [@Strainger](https://boards.straightdope.com/u/Strainger)\ **Post date:** [January 14, 2000, 5:28pm UTC](https://boards.straightdope.com/t/mass-weight-units/6266/6 "2000-01-14T17:28:00Z")
Actually, Jinx, it is the responsibilty of the engineer to use the correct units to obtain the correct results, although he may have to perform some conversions first. I’ve never heard of the g-c fudge thing. 1 kg[SUB]f[/SUB] is _not_ 1 N. A mass exerting 1kg[SUB]f[/SUB] has a mass of 1 kg[SUB]m[/SUB]. So, 1kg[SUB]f[/SUB] = 1 kg[SUB]m[/SUB] X 9.8 m/s[SUP]2[/SUP] (acceleration on Earth at S.L.) = 9.8 N. ---
**Author:** ![Strainger](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/strainger/32/244_2.png) [@Strainger](https://boards.straightdope.com/u/Strainger)\ **Post date:** [January 14, 2000, 5:31pm UTC](https://boards.straightdope.com/t/mass-weight-units/6266/7 "2000-01-14T17:31:00Z")
Also, Jinx, if you’re interested, you can download a handy-dandy conversion tool (I use it constantly) from [www.joshmadison.com](http://www.joshmadison.com) . It’s freeware so you can keep your credit card in your wallet. ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [January 14, 2000, 5:44pm UTC](https://boards.straightdope.com/t/mass-weight-units/6266/8 "2000-01-14T17:44:00Z")
Stainger, I follow you. But, haven’t you seen the (g/gc) term in equations? Seems to me it just helps adjust units of “lbm”. ---
**Author:** ![Strainger](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/strainger/32/244_2.png) [@Strainger](https://boards.straightdope.com/u/Strainger)\ **Post date:** [January 14, 2000, 5:50pm UTC](https://boards.straightdope.com/t/mass-weight-units/6266/9 "2000-01-14T17:50:00Z")
The g/g-c thing sounds like something I may have learned in my early aero classes, but don’t use anymore. I don’t think it does what I think you think it does, though. I remember some height-above-sea-level correction factor; maybe it’s related to that. I’ll have to take a look at my old textbooks when I’m at home and see if I can find what you’re asking about. And while I’m at it, I’ll try to answer curious george’s question from a while back about figuring out heights of buildings using a barometer. ---
**Author:** ![Strainger](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/strainger/32/244_2.png) [@Strainger](https://boards.straightdope.com/u/Strainger)\ **Post date:** [January 17, 2000, 2:02am UTC](https://boards.straightdope.com/t/mass-weight-units/6266/10 "2000-01-17T02:02:00Z")
Jinx, I found the answer regarding g[SUB]c[/SUB]. It is merely a conversion factor used in the F = ma equation to correct for nonconsistent units. From _Introduction to Flight_ by John D. Anderson Jr.: > [@](#): > > The English engineering system of units is another consistent set of units. Here the basic units of mass, length, time, and temperature are the slug, foot, second, and Rankine respectively. In this system, > > F=ma > > (1 pound) = (1 slug)(1 foot/second[SUP]2[/SUP]) (Eq. 2.5) > > The pound is a force defined such that it accelerates a mass of 1 slug by 1 foot/second[SUP]2[/SUP]. Note that in both systems, Newton’s second law is simply written simply as F=ma, with no conversion factor on the right-hand side. > > In contrast, a nonconsistent set of units defines force and mass such that Newton’s second law must be written with a conversion factor, or constant, as > > F = (1/g[SUB]c[/SUB]) x m x a > > A nonconsistent set of units frequently used in the past by mechanical engineers includes the pound or pound force, pound mass, foot, and second, whereby > > g[SUB]c[/SUB]=32.2 (lb[SUB]m[/SUB])(ft)/(s[SUP]2[/SUP])(lb[SUB]f[/SUB]) (Equation 2.6) > > In this system, the unit of mass is the pound mass lb[SUB]m[/SUB]. By comparing Eqs. (2.5) and (2.6), we see that 1 slug = 32.2 lb[SUB]m[/SUB]. A slug is a large hunk of mass, whereas the lb[SUB]m[/SUB] is considerably smaller, by a factor of 32.2. ---
**Author:** ![JonF](https://avatars.discourse-cdn.com/v4/letter/j/cab0a1/32.png) [@JonF](https://boards.straightdope.com/u/JonF)\ **Post date:** [January 17, 2000, 2:12am UTC](https://boards.straightdope.com/t/mass-weight-units/6266/11 "2000-01-17T02:12:00Z")
* * * ## Jinx, I found the answer regarding gc. It is merely a conversion factor used in the F = ma equation to correct for nonconsistent units. ## Exactly correct. And, as a long-time mechanical engineer, I think that slugs are an abomination. Lbf’s and lbm’s do it for me … {grin} jrf ---
**Author:** ![Strainger](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/strainger/32/244_2.png) [@Strainger](https://boards.straightdope.com/u/Strainger)\ **Post date:** [January 17, 2000, 2:27am UTC](https://boards.straightdope.com/t/mass-weight-units/6266/12 "2000-01-17T02:27:00Z")
> [@](#): > > Originally posted by JonF: > \*\* > Exactly correct. And, as a long-time mechanical engineer, I think that slugs are an abomination. Lbf’s and lbm’s do it for me … {grin} > \*\* I agree with you there, JonF! I guess on occasion I’ve used the 'g[SUB]c[/SUB] term without even realizing it, although the specs and property tables are usually set up to where I don’t even have to worry about it.