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Having trouble with weight versus mass

  1. Feb 17, 2010 #1
    F=ma and w=mg; with acceleration = gravity, then Force = weight.

    lb is a mass
    lbf is a force
    kg is a mass
    I would then assume that kgf is a force. But the units of force in SI units is Newton. So I would conclude that 1 kgf = 1 N. But it does not...

    When somebody says that something weighs 1 lb, then dividing by 32.2 ft/sec2 will define the mass in units of slug or lbm.

    When somebody says that something weighs 1 kg, they are not saying that it weighs 1 kg, but has 1 kg of mass, because if you multiply by 9.81 m/sec2, you get the units of Newton.

    I have gotten thoroughly confused on this. Please help.
     
  2. jcsd
  3. Feb 17, 2010 #2

    clem

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    Everything you say is right,, but I never heard of a kgf.
    Yes, the 'English' system, now used just about only in the US has two ways of doing it.
    `Pound` can be force and `slug` mass, but writing 1 pound=32 slugs can be confusing, unless you put the units into 32. The correct English is `The weight of a mass of 1 slug is 32 pounds.
    The other way is `pound` is mass and `The weight of a 1 pound mass is 32 poundals'.
    Poundal is the force unit I recall engineers using. My recollection is that physicists use slugs and engineers use poundals.
    SI allows (under pain of not being allowed to publish) only kg for mass and Newton for force, with `a mass of 1 kg has a weight of 9.8 Newton's' They no longer permit grams or cm, although seconds are still allowed. Then, it is incorrect to say (in any language)
    `I weigh 80 kg' (or`kilo' as they say, but only physics professors know that.
     
  4. Feb 17, 2010 #3

    mgb_phys

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    Correct, when somebody says they weigh 80Kg they mean they weigh the same as an 80kg mass.
    It is usually is less confusing in SI, where mass and force have different units - but it means some day-day statements are wrong
     
    Last edited: Feb 17, 2010
  5. Feb 17, 2010 #4
    the weight you have on a scale is force not mass. it measured the force of gravity on your mass. If you take that same scale to the moon the force will be less. Unless of course that is because scales take into account the force of gravity then give you your mass. Which is entirely possible. Do scales do that?
     
  6. Feb 17, 2010 #5
    Slugs ... they are my favorite unit.

    Thanks
    Matt
     
  7. Feb 17, 2010 #6

    mgb_phys

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    They aren't terribly useful as a standard - they shrink if you spill salt on them
     
  8. Feb 17, 2010 #7
    Yes, they will, unless you are using them in ANSYS. (That is the only place I use them.)
     
  9. Feb 17, 2010 #8
    Not as a standard, but you can throw them into a gun and get some good results.
     
  10. Feb 17, 2010 #9

    mgb_phys

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    Surely a little squashy?
     
  11. Feb 17, 2010 #10
    dont call me shirley
     
  12. Feb 17, 2010 #11

    Matterwave

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    kgf, or kilogram-force is the force of 1 kg accelerated at 9.8m/s. Thus, it is 9.8N.

    This way, when I say I weigh 60kg, I should really say I weigh 60kgf, and that my mass is 60kg.
     
  13. Feb 17, 2010 #12

    D H

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    First things first: Pounds are a unit of mass. The pound as a unit of mass have been around for a long time, long enough that there are several pounds. The avoirdupois and troy pound, for example. (A pound of feathers weighs more than a pound of gold, BTW. The troy pound is used for precious metals, the avoirdupois point for ordinary stuff like feathers.) Without any qualifier, the term 'pound' means the avoirdupois pound.

    Pounds are also units of currency. Back in the day, one troy pound of silver was worth one pound sterling. Nowadays that much silver is worth over 100 pounds.

    Finally, pounds are also a unit of force. The pound force is the force needed to accelerate a one pound mass by 1g (9.80665 m/s2 exactly, to mix measurement systems).

    What about the slug? Slugs are not an official part of the English system. They are a rather recent invention (100 years ago or so).

    Without any qualifier, the word 'pound' means the avoirdupois pound: A unit of mass. Symbol: lb. If you mean force, it is best to say pound-force (symbol: lbf).

    Or shoot, just go metric.
     
  14. Feb 17, 2010 #13

    mgb_phys

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    So if a rope says 6000lbs breaking strain does that mean it can lift a 6000lb mass or a 6000/32.2 lb mass?
     
  15. Feb 17, 2010 #14

    D H

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    A 6000 pound mass, or a 6000/32.2 slug mass.

    Interesting aside: The industry in which I work uses pounds for mass but uses slug-ft2 for moments and products of inertia.
     
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