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Pounds is a unit of mass or weight

  1. Sep 28, 2008 #1
    I've been looking around and some places tell me pounds is a english unit of weight and other tell me it's a unit of mass i.e lbf and lbm. I thought it was a unit of weight but i'm not so sure because you're able to convert kg to pounds right and kg is a unit of mass not weight.
     
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  3. Sep 28, 2008 #2

    madmike159

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    Its a unit of mass. 1 pound = 0.45359237Kg. People some times get weight and mass confused. e.g. " how much do you weigh" should be "what is your mass"
     
  4. Sep 28, 2008 #3

    PhanthomJay

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    The pound unit gets confusing, as there are several definitions related to separate terms like lbf, lbm, poundals, etc. For most physics problems using newton's laws, pound is defined as unit of force, used primarily in the USA. In the SI system, the force unit is the Newton. 1 kg weighs 9.8 newtons, or 2.2 pounds, more or less. That's a conversion factor used to equate mass with weight on planet Earth.
     
  5. Sep 28, 2008 #4
    I think it's both it's a unit of weight if it's pound-force and a unit of mass if it's pound-mass (lbf and lbm) lb generally refers to lbm
     
  6. Sep 28, 2008 #5

    PhanthomJay

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    CORRECT
    INCORRECT. lb or lbs (pound or pounds) refers to a unit of force or weight, not mass. I told you it would be confusing! If a man who weighs 200 pounds hangs from a rope, the rope tension is 200 lbs, a force unit.
     
  7. Sep 28, 2008 #6

    LURCH

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    A person who weighs 200 bls on Earth would weigh 33 lbs on the Moon, so "lbs" is a unit of weight, not mass.

    Holtvg; do you live in the US? (just curious; because if I lived anywhere else, I don't think I would bother learning the Empirial system.)
     
  8. Sep 28, 2008 #7

    D H

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    Correct - and a unit of money as well. For a long time, one pound (mass) of silver was worth one pound (sterling) in money, by fiat. Now that we have switched to fiat money the old fiat no longer applies.

    holtvg is CORRECT, PhantomJay. lb, without an additional qualifier, indicates the pound (mass). The National Institutes of Standards and Technology is the standard bearer in this regard. See, for example, their conversion tables.

    Sorry about the bad puns. I couldn't resist.
     
  9. Sep 28, 2008 #8

    PhanthomJay

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    When did that standard come out? As far as I'm concerned, I weigh 200 lbs, a force unit. Now I might weigh 200lbf, but I've never, or seldom, seen it that way. And either in layman's terms or in physics, I've never seen seen lb used to designate mass. They're going to have to change a lot of text books in engineering statics if this is the case, but I doubt it will ever happen. The unit of mass in the USA system is the seldom used 'slug', which weighs 32 pounds (lbs) on Earth. A net force of 1 pound (lb) will give an object with a mass of 1 slug, an acceleration of 1ft/sec^2, per newton 2. I have no idea how to apply newton 2 if the mass is given in lbm or 'lb'. I'd flunk Physics 101 for sure that way!
     
  10. Sep 28, 2008 #9

    Janus

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    Here's the thing. There are two ways to express both the MKS and FPS systems, absolute and gravitational. In absolute terms, the unit of mass in the FPS system is the lbm and the unit of force is the poundal. Under the same terms, the Kilogram is the unit of mass and the newton the unit of force for the MKS system. In gravitational terms, the slug is the FPS unit of mass, the lbf[/sub} is the FPS unit of force, the kgf is the MKS unit of force, and the kg-sec²/m is the MKS unit of mass.

    Commonly in the US, you will compare lb to kg (1 kg = 2.2lb for instance) and the kg is considered a unit of mass. You wouldn't directly compare a force to a mass, you compare a a mass to a mass.
     
  11. Sep 28, 2008 #10
    Yes i live in the us don't won't to learn the imperial system just curious as i was confused about that part. So what's the difference between a poundal and lbf
     
  12. Sep 28, 2008 #11

    PhanthomJay

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    If you live in the US and want to be a civil or structural engineer, or want to buy a pound of apples, you're not going to make it on either count. 50 years ago they told me I had to convert to metric. I'm still waiting, and I'm in no hurry!
    I don't know and I don't care!!! (If I had to buy a poundals worth of apples, I'd be the one in BIG trouble!) (I'd like to make a smiley face, but I don't know how, let me try: :smile)
     
    Last edited: Sep 28, 2008
  13. Sep 28, 2008 #12

    D H

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    Since 1832, officially. Unofficially, far before that.
    That is because in lay and legal terminology, weight is synonymous with mass. Since the NIST sets the legal standards of weights and measures in the US, it is governed by legal terminology.
    No, it is not. The legal unit of mass in the US is the pound. The slug is a seldom used derived unit.
    Newton did not say that force is mass times acceleration (F=ma). What he said instead is that force is proportional to the product of mass and acceleration, or F=kma. (Actually he didn't say that either; he said "Mutationem motus proportionalem esse vi motrici impressae, et fieri secundum lineam rectam qua vis illa imprimitur," or "The alteration of motion is ever proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.") In Physics 101, we ignore variations in mass, so change in momentum is simply mass times acceleration. Thus Newton's second law is, ignoring changes in mass, F=kma.

    The metric system was designed so that the proportionality constant has a numerical value of 1, exactly. If you measure mass in pounds and force in pounds-force, the proportionality constant is 1/32.17405 lbf-sec2/lb/ft.

    Or you could use pounds force and slugs, which are defined as 32.17405 pounds, and then you have F=ma.

    Or you could use pounds mass and poundals, which are defined as 1/32.17405 pounds-force, and then you again have F=ma.

    Or you could just view us Americans as incredibly stupid and use metric units, and then you again have F=ma.
     
  14. Sep 28, 2008 #13

    PhanthomJay

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    I'm glad I'm a science guy and not a lawyer!
     
  15. Sep 28, 2008 #14

    Janus

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    A poundal will accelerate a 1 lb mass at 1 ft/sec²
    A lbf will accelerate a 1 slug mass at 1 ft/sec²

    IOW, a poundal is about 1/32 the force of a lbf
     
  16. Sep 28, 2008 #15

    Dale

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    Are you sure about that? Because as of late 90's engineering professors were still teaching that a pound is a unit of force. Also, US auto manufacturers still quote torque as lb.-ft., which indicates that the auto industry thinks that lb. = lbf. != lbm, and since the specification sheets are legal documents I would think they would need to use the right units.
     
  17. Sep 28, 2008 #16

    rcgldr

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    < General usage of pound

    The english unit for torque is foot pounds, where pound is a force.
    How much someone weighs (a force) is also pounds, but weight force is relative to mass.

    As mentioned, the English unit of mass is a "slug".
    Gravity is usually defined as 32.174 ft / sec^2
    1 pound = 1 slug foot / second^2.
    So a slug = 31.274 pound(mass).

    In SI units, describing a mass by it's weight could be express as Newtons, but this is rarely done. I'm not sure of the history of why using pound-mass became common.
     
  18. Sep 28, 2008 #17

    LURCH

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    So, if I weigh 200 lbs on Earth, I will weigh 200lbs on the Moon? That is not what I learned since ellementary school.
     
  19. Sep 28, 2008 #18

    LURCH

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    So, if I weigh 200lbs on Earth, I weigh weigh 200lbs on the Moon?
     
  20. Sep 28, 2008 #19

    Janus

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    Well, it depends on what type of scale you use. If you use a spring scale, it actually measures force, which it displays as lbs assuming an Earth surface force of gravity, and it will read about 33 lbs. However, if you use a beam balance scale which measures mass, it will read 200 lbs.
     
  21. Sep 29, 2008 #20

    D H

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    A pound is both a unit of mass and a unit of force. If engineering professors don't teach that they are wrong. The National Institute of Standards and Technology, formerly the National Bureau of Standards, formerly the Coast and Geodetic Survey Office of Weights and Measures, defines standards in the U.S., not engineering professors.

    NIST defines the avoirdupois pound to be exactly 0.45359237 kilograms and the pound force to be exactly 0.45359237 kg × 9.80665 m/s² = 4.4482216152605 Newtons. When not qualified, the term "pound" means the avoirdupois pound. For more info see NIST Special Publication 811.

    Officially, that unit is lbf-ft, not lb-ft. See NIST Special Publication 811.
    No, it is not. Before the official adoption of the metric system in 1893, the official unit of mass was the grain. One (avoirdupois) pound was (and still is) exactly 7000 grains, while one troy pound was (and still is) exactly 5760 grains. Since 1893 (the Mendenhall Order), the official unit of mass is the kilogram. The Mendenhall Order was supplanted in 1959, slightly changing the definition of the pound mass.

    Commerce. Dumping the contents of a one pound can of peas on a spring scale calibrated for Earth standard gravity will yield a weight of 1.0016 pounds (force) in Nome, Alaska but only 0.9966 pounds (force) at the Mauna Kea observatory. That isn't good for commerce. The contents of a one pound can of peas has the same legal weight in Nome as it does at the observatory -- one avoirdupois pound, or one pound for short. Legally, weight is synonymous with mass, not force.
     
    Last edited: Sep 29, 2008
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