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?

 

[electricpete]

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.

That's exactly what I do....

Except I never thought to call 'em as hillbilly units until now. It's gonna be tough to get that out of my mind LOL.

=====================================
(2B)+(2B)' ?

 

[rb1957]

The NA aerospace industry is the last hold out for the imperial system. I'm not saying exclusively (I'm sure there are some to correct me) .. I'm saying 90% imperial (and 10% funny).

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

 

[JohnRBaker]

Yes, I found that to be true when I was still supporting our CAD software at the various aerospace companies and their suppliers, Imperials units ruled the day and when you did find something specified in Metric units, it was often an Imperial dimension converted, which meant that it was not a nice, clean numeric.

Of course, we also served the auto industry and they had made the move to the SI system years ago.

With that in mind, I have a bit of trivia; even back when the American automobile industry was still exclusively using Imperial units, there were always at least six and sometimes eight, Metric threaded holes in the car. You see, spark-plugs, being a European (Germany) invention, they had historically ONLY come with Metric threads, so as for the America car people, their fate was already set when they decided to just go with it and NOT develop and manufacture spark-plugs with Imperial threads. The handwriting was therefore already on the wall, as it were.

But for a bit of reverse trivia, despite the fact that the specs for the tread width/ratio of tires are given in Metric units, the actual dimensions of the wheels that they're mounted on are still specified in Imperial units.

It's a complex, and often contradictory, world that we Engineers are forced to live and work in ;-)

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]

But for a bit of reverse trivia, despite the fact that the specs for the tread width/ratio of tires are given in Metric units, the actual dimensions of the wheels that they're mounted on are still specified in Imperial units.

I'm looking for a RAV4 Hyrid, and a car coming from Japan, ostensibly all metric, does indeed have either 17-inch or 18-in wheels, but I think the lug nuts are metric-threaded

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

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[dik]

I work the other way... in Imperial and convert to SI... just an old dinosaur. About 90% of the stuff I do is still Imperial.

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

-Dik

 

[3DDave]

I really like tire dimensional specifications - not. What I want is the mating rim diameter, a design rim width, overall tread width, and the OD/Perimeter.

What do I get? Width in millimeters, ratio of some height to some width, and then mating rim diameter. The width in millimeters is mostly a mold dimension, includes the raised letters and varies some on installation with the rim width. So why is it 3 significant digits?

I do not like this Sam I Am, I like it less than Green Eggs and Ham.

 

[rb1957]

hadn't thought about wheel rims ... of course 18" rims !! not 460mm, or standard sizes 500mm/450mm/400mm ??

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

 

[NedGan76]

lbf is for weight which is a force (= mass x acceleration), and lbm is for mass. So, they are not equal in the same way as Newtons are not equal to killograms.

 

[drawoh]

NedGan76,

No. A pound (lb) is a unit of force. Period.

You can be sloppy with this stuff doing static calculations, but the moment things start accelerating, you need to know where to insert G in your equations.

[tt]<action>Drawoh stares at NegGan76 as if he were a space alien.</action>[/tt]

--
JHG

 

[rb1957]

why I smile at reports that say "the mass of the unit is X lbf" ... I guess "weight" isn't sciency enough ?

or worse use "lb" as a unit of mass and so confuse the matter completely ... do you mean the mass is X lbm, or the weight is X lbf ?

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

 

[Denial]

I'm with NedGan76 on this.[&nbsp;] Where I come[&nbsp;]from / came[&nbsp;]from, "lb" is used for mass, and "lbf" is used for force.[&nbsp;] If absolute clarity is required then "lbm" is sometimes used, but this is rare.

The current "discussion" seems to hinge on geography rather than physics.

 

[3DDave]

Considering further - in-lbf and ft-lbf are for driving nuts.

 

[electricpete]

I have no doubt all the responders have a firm grasp of these things. In the US, we learned it in high school. Here on eng-tips we've hashed it over a lot of times (probably approaching Avogardo's number of times).

Just to circle back to op (in case he ever comes back here)

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

(B) is true.

(A) is illogical, the results of both sides of an equation must be the same dimension. You have two different dimensions force on the left and mass on the right. You can correct A by saying that 1 lbm weighs 1 lbm (under standard gravity).

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?

Yes, that's it.

Or does it have a mass of 10/32.2 = 0.31 lbm = 0.31 * 453.6 = 140.8 g?

No. If you want to include the 32.2 number when finding weights in lbf for forces in lbm (or vice versa), it gets included twice and cancels out.

Your barbell weighs W = 10 lbf
The mass is M = W / g = = 10 lbf / (32.2 ft/sec^2) = (10/32.2) ( lbf*sec^2 / ft )
Convert the units on the right using the relationship: 1 lbf = 1 lbm * 32.2 ft/sec^2:
M = (10/32.2) ( lbf*sec^2) / (ft) * [(lbm * 32.2 ft/sec^2)/lbf]
After cancelling like terms in numerator and denominator you're left with
M = 10 lbm


=====================================
(2B)+(2B)' ?

 

[dik]

453.6g to 1lb...

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

-Dik