lbm and lbf are driving me nuts! 8

[turkey937123]

I am working on a job where I have to use both English units and SI units. I have only used English units up until this point.

I feel dumb asking this question, but what is the conversion between lbf and grams?

This is confusing me, because my understanding was that lbf = lbm (32.2 ft/s^2). In other words, lbf and lbm are not equal. However, I am finding a conversion factor of 453.6 for both lbf to grams and lbm to grams:

https://www.convertunits.com/from/pound+mass/to/grams

https://www.convertunits.com/from/pound-force/to/grams

That makes it sound like 1 lbm = 1 lbf.

So does 1 lbf=1lbm, or does 1lbf = 1lbm (32.2 ft/s^2)? I am finding both online, and it is driving me crazy.

To give a concrete example, if I have a barbell that weighs 10 lb on the scale (so 10 lbf), does that barbell have a mass of 10 lbm and therefore 4536 g? Or does it have a mass of 10/32.2 = 0.31 lbm = 0.31 * 453.6 = 140.8 g?

 

[Compositepro]

One lb mass exerts one lb force in a gravitational field of one G. Gravity does vary slightly by location on the earth.

 

[JohnRBaker]

When you're sitting on the Earth and nothing's moving, you can safely assume that lbf = lbm, as described in this rather complex, but still well written treatise on the subject:

https://engineering.stackexchange.com/questions/2300/pound-force-lbf-vs-pound-mass-lbm

John R. Baker, P.E. (ret)
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[IRstuff]

Just to be clear,

kg = 2.2046 lbm
kg * g = 2.2046 lbf

so,

lbm * g = lbf

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[cowski]

Don't forget our friend the slug.
1 slug = ~32.2 lbm

 

[TheTick]

I don't even muck about with The King's system if I can help it.

Convert the problem to metric. Solve in metric. Convert the answer to hillbilly units when finished.

I started doing that in navy nuc school and all the way through college. Spared me many conversion errors.

Even the Apollo program worked this way.

 

[3DDave]

The slug is truly a good buddy unit. 1 lbf = 1 slug * 1 (ft/sec)^2 One slug of mass weighs ~32.2 lbs at 1 g.

By the time one had determined that a slug is needed in a calculation one has already decided if mass or force is the key element.

 

[drawoh]

turkey937123,

This one is easy. Pounds mass and kilograms force are abomination. Don't use them. Pounds are units of force which convert to Newtons. Kilograms are units of mass which convert to slugs. Slugs can be ignored if you substitude w/g for m.

Your basic problem is knowing where to stick g in all your acceleration equations. English units actually are not a problem. Use the SI equations, and treat pounds as units of force, not mass. If people talk to you about lbm and kgfs, stare at them blankly like they are a space alien.

--
JHG

 

[IRstuff]

Convert the problem to metric. Solve in metric. Convert the answer to hillbilly units when finished.

That's where Mathcad, Smath, and CalcPad come in; conversions are transparent and on the fly, no fuss, no muss. You can mix whatever units you like, and Mathcad either has them built-in, or you can pre-define them. My HS AP Pysics teacher loved furlongs/fortnight.

Arguably, Mathcad does convert EVERYTHING to SI first, and all calculations are done in SI, and you can select the units you want for the output, so it's doing what Tick suggests, but you don' even have to do the individual conversions; easy-peasey

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[rb1957]

To give a concrete example, if I have a barbell that weighs 10 lb on the scale (so 10 lbf), does that barbell have a mass of 10 lbm and therefore 4536 g? Or does it have a mass of 10/32.2 = 0.31 lbm = 0.31 * 453.6 = 140.8 g?

Ooppss ...
1 lbf = 1 slug*1 ft/sec^2, 1 slug = 32 lbm
1 lbf = 1 lbm*32 ft/sec^2 (1 lbf is the weight of 1lbm, = 32 lbm*ft/sec^2)
1 kgf = 1 kg*9.7m/sec^2 = 9.7N (the weight of 1 kg, mass)

masses (2.2 lbm or 1kg) have weights (2.2lbf or 1 kgf, or 9.7N)

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[IRstuff]

To give a concrete example, if I have a barbell that weighs 10 lb on the scale (so 10 lbf), does that barbell have a mass of 10 lbm and therefore 4536 g? Or does it have a mass of 10/32.2 = 0.31 lbm = 0.31 * 453.6 = 140.8 g

Common scales are designed to provide the mass of the object being weighed, regardless of whether it's lbm or kg. Nevertheless, if you can't tell that something is 10 lbm when you lift it to put it on the scale, there's a bigger issue at play ;-)

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[dik]

that's why in unit definitions for SMath, I have the following:

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[3DDave]

rb1957 - read what I wrote again. Sure, I changed it after, but it's now correct.

"32 lbf = 1 lbm*32 ft/sec^2 "

Wait, what?

 

[IRstuff]

that's why in unit definitions for SMath,

Huh, so psi and psf aren't built-in SMath units? That's disappointing; they're all built-in Mathcad units.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[dik]

for lb measurements, it defaults to 'mass'... it gets interesting for density like issues...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dvd]

Seems like some sort of poetry is in order -

A Haiku written on the occasion of responding to Eng-Tips imperial units posting

pounds-mass has same weight
as pounds-force on this planet
the king's units rock!

 

[TugboatEng]

It's funny watching people complain about lbs and slugs. What about Kgs and Newtons? Heck, you don't even need to remember the decimal if you're using two significant figures when using imperial units. Simply 32.

 

[BrianPetersen]

I am another who does all important calculations in SI units, even if it means converting the original units to SI, doing the calculation, and converting it back.

 

[kjoiner]

I don't do them often, but I do heat or thermo calculations using SI units and convert back if necessary.

Kyle

 

[IRstuff]

Again, Mathcad, SMath, or CalcPad, and let the program sort it out; no need to have to keep a cheat sheet of unit conversions.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm