Comments on: What the Heck’s the Difference Between Pound-Force and Pound-Mass? http://engineerography.com/2009/03/what-the-hecks-the-difference-between-pound-force-and-pound-mass/ Studying and writing about everyday engineering, since 2009. Sun, 29 Jan 2012 21:37:10 +0000 hourly 1 http://wordpress.org/?v=3.1 By: Taylor http://engineerography.com/2009/03/what-the-hecks-the-difference-between-pound-force-and-pound-mass/#comment-1210 Taylor Wed, 09 Nov 2011 18:34:39 +0000 http://engineerography.com/?p=360#comment-1210 Thank you for posting this! clears up soooo much confusion... My teacher who's a working engineer explained this to me wrong and said anytime something is talking about lbm, I need to multiply it by 32.174 ft/s^2. My answers were not matching up with the textbook, he had me believing the text book was wrong! It would help if my textbook explained the english system in this way... Up until this point I was like your friends, converting english units into metric units... I now have a much more clear picture of how these work. Thank you Tom for the equation F = mg/gc as this is a useful way to think about it. my question is now, When would you simply multiply lbm by 32.174 ft/s^2 to end up with an accurate lbf? Thanks yall! Thank you for posting this! clears up soooo much confusion… My teacher who’s a working engineer explained this to me wrong and said anytime something is talking about lbm, I need to multiply it by 32.174 ft/s^2. My answers were not matching up with the textbook, he had me believing the text book was wrong! It would help if my textbook explained the english system in this way… Up until this point I was like your friends, converting english units into metric units… I now have a much more clear picture of how these work. Thank you Tom for the equation F = mg/gc as this is a useful way to think about it. my question is now, When would you simply multiply lbm by 32.174 ft/s^2 to end up with an accurate lbf? Thanks yall!

]]> By: Nima http://engineerography.com/2009/03/what-the-hecks-the-difference-between-pound-force-and-pound-mass/#comment-1208 Nima Sun, 16 Oct 2011 04:23:36 +0000 http://engineerography.com/?p=360#comment-1208 awesomeeeeeeeeee.Thanks! awesomeeeeeeeeee.Thanks!

]]> By: Tom http://engineerography.com/2009/03/what-the-hecks-the-difference-between-pound-force-and-pound-mass/#comment-1198 Tom Wed, 02 Feb 2011 20:28:51 +0000 http://engineerography.com/?p=360#comment-1198 Ignore my last comment/question. I studied this some more and this website actually has the correct information, it's the ehow.com website that's wrong. Apparently you cannot simply use Newton's 2nd law F=ma as simply lbf=lbm*g as the ehow.com site does. I now understand there's a proportionality constant "g sub c" or gc which is 32.17 lbm ft/lbf s^2 which must be used. So Newton's 2nd law is written F=ma/gc when using imperial (English) units. Thus F=mg/gc, and g and gc will cancel each other out. If you simply use F=ma as lbf=lbm * g then there is no way that lbf = lbm, but using F=ma/gc allows lbf = lbm when g=32.17. Now I comprehend. And the ehow.com website is the incorrect one. Ignore my last comment/question. I studied this some more and this website actually has the correct information, it’s the ehow.com website that’s wrong. Apparently you cannot simply use Newton’s 2nd law F=ma as simply lbf=lbm*g as the ehow.com site does. I now understand there’s a proportionality constant “g sub c” or gc which is 32.17 lbm ft/lbf s^2 which must be used. So Newton’s 2nd law is written F=ma/gc when using imperial (English) units. Thus F=mg/gc, and g and gc will cancel each other out. If you simply use F=ma as lbf=lbm * g then there is no way that lbf = lbm, but using F=ma/gc allows lbf = lbm when g=32.17. Now I comprehend. And the ehow.com website is the incorrect one.

]]> By: Tom http://engineerography.com/2009/03/what-the-hecks-the-difference-between-pound-force-and-pound-mass/#comment-1197 Tom Tue, 01 Feb 2011 15:13:38 +0000 http://engineerography.com/?p=360#comment-1197 "You can take the following to be true: 1 slug = 32.17 lbm" I believe this is not correct. Looking at other lbm/lbf sites and http://www.onlineconversion.com/weight_all.htm, I find 1 lb, lbs = 0.031 080 950 038 slug 1 pound = 0.031 080 950 038 slug 1 pound-force = 0.031 080 950 038 slug 1 slug = 32.174 048 695 pound-force, etc. which makes the following statements incorrect also: "Here’s the rub, 1 lbf is the same as 1 lbm if the acceleration is equal to gravity, which is 32.17 ft/s^2. Weird! So you can weigh yourself and the scale reads both pound-force and pound-mass at the same time." "We weigh Bob and find that his weight is 160 pounds-force (note that this is the weight directly off the bathroom scale, also 160 pounds-mass if gravity at the specific location is 32.17 ft/s^2)" My understanding of the relationship between lbm and lbf (substantiated by numerous other web sites such as http://www.ehow.com/way_5163173_pound-force-pound-mass-conversion.html for example show the above 2 statements to be incorrect. But, I'm not a mechanical engineer so maybe my understanding is faulty. If I'm wrong, then I'm obviously totally confused. Would someone please look into this and clarify? “You can take the following to be true: 1 slug = 32.17 lbm”
I believe this is not correct. Looking at other lbm/lbf sites and http://www.onlineconversion.com/weight_all.htm, I find
1 lb, lbs = 0.031 080 950 038 slug
1 pound = 0.031 080 950 038 slug
1 pound-force = 0.031 080 950 038 slug
1 slug = 32.174 048 695 pound-force, etc. which makes the following statements incorrect also:

“Here’s the rub, 1 lbf is the same as 1 lbm if the acceleration is equal to gravity, which is 32.17 ft/s^2. Weird! So you can weigh yourself and the scale reads both pound-force and pound-mass at the same time.”
“We weigh Bob and find that his weight is 160 pounds-force (note that this is the weight directly off the bathroom scale, also 160 pounds-mass if gravity at the specific location is 32.17 ft/s^2)”

My understanding of the relationship between lbm and lbf (substantiated by numerous other web sites such as
http://www.ehow.com/way_5163173_pound-force-pound-mass-conversion.html
for example show the above 2 statements to be incorrect. But, I’m not a mechanical engineer so maybe my understanding is faulty. If I’m wrong, then I’m obviously totally confused. Would someone please look into this and clarify?

]]> By: Matt http://engineerography.com/2009/03/what-the-hecks-the-difference-between-pound-force-and-pound-mass/#comment-1188 Matt Tue, 26 Oct 2010 18:00:48 +0000 http://engineerography.com/?p=360#comment-1188 I want to second Randy's comment. I missed the first week of my material balance class and was thus quite confused when I first encountered pounds mass/force. This post was helpful in understanding fully. Thanks! I want to second Randy’s comment. I missed the first week of my material balance class and was thus quite confused when I first encountered pounds mass/force.

This post was helpful in understanding fully. Thanks!

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