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 Message Boards » » lbm vs lbf Page [1]  
ncsufox07
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Can someone run through this in terms of F=ma or W=mg for me?

6/25/2007 3:27:38 PM

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basically a lbf is a lbf no matter where you are, earth, moon, etc

1 lbm is the amount of mass that has a force of 1 lbf on earth

1 lbm would be different on the moon

you use the gravitational constant (32.2 lbm*ft/lbf*s^2) to convert between them

it is stupid

if you want to use F = m * a

then 1 lbf = 1 lbm * 32.2 ft/s^2

the kgf exists as well (people say that something weighs xx kg, which is wrong because kgs measure mass not weight [force])



[Edited on June 25, 2007 at 3:38 PM. Reason :

6/25/2007 3:31:42 PM

guth
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slugs sounds cooler

6/25/2007 8:27:30 PM

CharlieEFH
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Quote :
"it is stupid"


is not

it would be stupid if everyone understood the difference and it didn't need explaining because it was so obvious

but obvioulsy its not so obvious....

6/25/2007 8:51:48 PM

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Quote :
"slugs sounds cooler"


thats part of the problem right there

in metric the force is the derived unit (1 N = 1 kg * m / s^2)

whereas in the english system the slug is the derived unit (1 slug = 1 lb * s^2 / ft)

Quote :
"is not

it would be stupid if everyone understood the difference and it didn't need explaining because it was so obvious

but obvioulsy its not so obvious...."


yeah, its still stupid, because there are already units in place to measure either the mass or force in question

there is no reason for bastardized units

6/25/2007 9:27:00 PM

iulus
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Quote :
"basically a lbf is a lbf no matter where you are, earth, moon, etc

1 lbm is the amount of mass that has a force of 1 lbf on earth

1 lbm would be different on the moon"


Granted the last time I took a Newtonian physics class was back in 2003 so I might be a little rusty, but isn't the mass (and thus, lbm) the property of an object that remains constant no matter the gravitational condition? Using Newton's second law, F = m * a, as the acceleration is varied, the force changes, not the mass. Same holds true for W = m * g, since it's just a rewrite of Newton's second law. As the acceleration due to gravity changes, the mass doesn't change, the weight (lbf) does.

[Edited on June 25, 2007 at 10:02 PM. Reason : typo]

6/25/2007 10:02:15 PM

CharlieEFH
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Quote :
"but isn't the mass (and thus, lbm) the property of an object that remains constant no matter the gravitational condition? "


yep

like i said...obviously its not so obvious...

6/25/2007 10:16:01 PM

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see why it is a stupid system

i fucked it up

yes, the lbm should be constant



[Edited on June 25, 2007 at 10:37 PM. Reason :

6/25/2007 10:23:24 PM

CharlieEFH
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obviously

6/25/2007 10:47:03 PM

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well shit happens

i was really adamant about it because i hate the system

6/25/2007 10:54:52 PM

Lowjack
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real systems LOVE to use bastardized units

6/26/2007 12:35:41 AM

ncsufox07
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Well I muddled my way through english unit problems through school and now I'm in the real world and actually having to use it so I'm trying to figure it out.

6/26/2007 8:07:47 AM

wizzkidd
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Iulus you've just illustrated WHY the system is stupid. Which unit is constant if you don't use the earth. 1lbf is only correct on earth. Physicists don't use it. Engineers use it b/c a 1lb object (on earth) has a mass of 1lbm and we're lazy, but I fucked hated it when I was in undergrad.

nscufox, just realise that there's a factor of 32.2ft/s^2 in there and figure it out that way.

6/28/2007 10:17:28 PM

CharlieEFH
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anyone who's having a problem differentiating between the concepts of mass and force should rethink being an engineer...

6/28/2007 10:36:01 PM

ncsufox07
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Good thing the concepts of mass and force aren't the problem then, but units.

6/29/2007 10:04:21 AM

CalledToArms
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^ yep. the concept of mass and force is like HS physics..but first dealing with lbm and lbf in undergrad is a complete pain for a lot of people trust me.

its not the same

6/29/2007 1:29:59 PM

CharlieEFH
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units are just bookkeeping, nothing hard about them

this entire thread gets a

6/29/2007 3:56:38 PM

LimpyNuts
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Quote :
"1 lbm would be different on the moon"
Not true at all. A lbm is the mass that exerts 1 lbf under 1g. The lbm does not vary under different gravitational conditions.

6/30/2007 2:08:29 AM

3 of 11
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If we were meant to use the metric system, Jesus would have had 10 disciples.

7/1/2007 12:55:26 AM

wizzkidd
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^LOL best line of this thread.

CharlieEFH, I don't think anyone here is confused on the difference between a mass and a force.

7/1/2007 11:03:06 AM

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eh, i had the symbols backwards in my head in my first post

7/1/2007 1:40:15 PM

 Message Boards » Study Hall » lbm vs lbf Page [1]  
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