Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Why is 1 lbm considered equal 1 lbf?

  1. Feb 4, 2017 #1
    My understanding is that 1lbm = l lbf/32.174 ft/s^2 = 3.1081*10^-2 (lb*s^2)/ft (slug)

    My textbook suddenly claims that 1 lbf is equal to 1 lbm. This doesn't fundamentally make sense to me because one unit is mass and the other is force (which has an acceleration constant).

    Thanks for helping me understand
     
    Last edited: Feb 4, 2017
  2. jcsd
  3. Feb 4, 2017 #2
    They aren't the same; one is a mass and the other a force.
    Unfortunately, the same confusion showed up with the kilogram: we have the mass kilogram and the force kilogram (now largely replaced with the Newton as the unit of force).
    The quantities differ in the gravity acceleration, that's why you invent the slug in a way that when you apply a force of one pound you get an acceleration of 1ft/s2. A slug is a big chunk of mass.Conversely, when a one force pounds acts on a mass pound we obtain an acceleration equal to g (32.2 ft/s2)
    When a was a teenager I spent long hours memorizing the different conversion factors. Now I hardly remember them and I use SI units.
     
  4. Feb 4, 2017 #3

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Textbook is wrong if it is worded that way. 1 lbm has a weight of 1 lbf on planet Earth. It's mass is as you state , in slugs. Always use slugs as the mass unit in calculations involving mass. The pound-mass is seldom used,
     
  5. Feb 4, 2017 #4
    I have never heard of a "force kilogram". And I come from a metric country.
     
  6. Feb 4, 2017 #5
    *groans*

    I would say, never use slugs for anything.
     
  7. Feb 4, 2017 #6

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    W
    well I guess you don't come from the USA. If imperial units of force and length (poinds and feet) are used, which is the norm in many engineering disciplines, then youust use slugs as the mass unit. Usually we talk force units of pounfds,and must divide it by 32 to get the proper mass unit in problems involving mass such as when calculating accelerations in ft/sec^2, or for frequency or seismic calculations.
     
  8. Feb 4, 2017 #7
    I know that that's what's happening, but I would hope you agree that it shouldn't be that way. The moment derived units come into play, anything but SI is a waste of time. Reality is, you aren't hireable for an internationally operating engineering company if you can't do SI.
     
  9. Feb 4, 2017 #8

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Many agree that the US should get in step with the rest of the world and convert to SI. But the fact is that is not the case now, and units of pounds and feet will be around for decades to come. So here, if you apply a force of ten pounds to a mass of one slug, its acc is 10 ft/sec^2. But would be a waste of time to convert to kilos and newtons and get results in meters per sec squared and then convert back again to ft per sec squared. If you want to get a job here you need to know customary US units.
     
  10. Feb 5, 2017 #9
    When I was a teenager, the Newton hadn´'t won "the battle of forces" yet and we had the force kilogram (the weight of the famous cylinder) and the mass kilogram (the mass of said cylinder). As a result, we had a unit of mass conceptually similar to the slug. A big mess I don´t miss.
    As for the Imperial system, when I visited England I found the only remainder were some distances quoted in miles. They sell coffee by grams and milk by liters. Perhaps the name should be changed to US system.
     
  11. Feb 6, 2017 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Which decade was that? (and where?) I started at School (UK, 1950s) with ft,lbs and poundals (yuk), then moved on to cgs (dynes, ergs etc). Finally ended up (1960s) with mks, which became regularised as SI. I never remember anyone talking of a force kilogram. What a ghastly idea.
     
  12. Feb 6, 2017 #11
  13. Feb 6, 2017 #12

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    I guess that Mr Scales my Physics master and hero (and possibly the Oxford and Cambridge Joint Examination Board to) decided to avoid such a horrible unit. My physics education was never aimed in the direction of Engineering.
     
  14. Feb 6, 2017 #13
    I actually have to somewhat revise my initial statement about the kilogram-force. In the country I grew up in it had the name "kilopond', and yes, I have heard that name before. It has a distinct "twirly moustache" feeling to it though.
     
  15. Feb 8, 2017 #14
    Last night on YouTube I saw a video from 1972 on how America would soon be a metric country. :smile:
     
  16. Feb 8, 2017 #15
    I have asked this before but I don't remember getting an answer: When someone asks your weight, do you answer "70 kilos" or "685 newtons" ?
     
  17. Feb 8, 2017 #16

    jbriggs444

    User Avatar
    Science Advisor

    The question "how much do you weigh" is reasonably interpreted as "what is the mass of your body". That is the quantity that is of importance to most physicians (other than orthopedists dealing with ankle, knee and hip problems). It is also the quantity that is measured in a doctor's office on the balance scales there. It is the quantity that is nominally measured by your bathroom scales, even though the displayed value is typically determined through the use of a force proxy. In the grocery store, calibration is done so that the displayed quantity is not just nominally equal to the mass of the object being weighted, it is actually equal to the mass of that object despite the use of a force proxy.

    Despite what most first year physics textbooks teach (or taught in my day), in the U.S. the "pound" is legally standardized as a unit of mass and "net weight" is legally standardized as an assertion about mass. In the grocery store, "weight" is mass. In the physics classroom, "weight" is force.
     
  18. Feb 8, 2017 #17
    So, your answer is "xx kilos"?

    I understand all the rest of your post and I don't disagree with any of it.
     
  19. Feb 8, 2017 #18

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Yes. And if you went into a greengrocer's shop and asked for 20N of potatoes he/she would give you a funny look. But there is a difference between a Scientific discussion and a system of proper units and the way those units are used by the general public. I could give you an almost endless list of how Science is mis-used by non-Scientists. We just have to grit out teeth and avoid being that naughty amongst ourselves.
     
  20. Feb 8, 2017 #19

    jbriggs444

    User Avatar
    Science Advisor

    Yes. At least that would be my answer if I lived and worked in a country whose customary system of units and measures included kilograms and newtons.

    Where I actually live, I would give the answer as [too many] pounds and not worry about the fractional percentage point of ambiguity in the answer.
     
  21. Feb 8, 2017 #20
    OK thanks you guys. My curiosity is not about "scientific discussions" I was really interested in what terms are in "everyday" use in SI countries. I half expected the answer to be "11 stone."
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Why is 1 lbm considered equal 1 lbf?
  1. Lbm or lbf? (Replies: 4)

Loading...