Mass & Weight Units

Factual Questions
1

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!

2

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.

3

PS, I had to read the OP several times until I understood the question. You should have phrased itL

4

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].

5

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.)

6

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.

7

Also, Jinx, if you’re interested, you can download a handy-dandy conversion tool (I use it constantly) from www.joshmadison.com . It’s freeware so you can keep your credit card in your wallet.

8

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”.

9

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.

10

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.:

11

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

12

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.