Def'n: mass vs. weight / kg vs. lbs

  1. DaveC426913

    DaveC426913 16,439
    Gold Member

    My friend, who has been researching basic physics, is saying the following, which I frankly had never heard of:

    We use metric and imperial measurements interchangably (I weigh 82kg or 180 lbs, same diff) but they are not at their essense the same thing.

    Pounds measure weight - our weight here on Earth (an effect of the gravitational pull of the Earth), whereas kilograms are intended to measure mass (a property of the number of atoms in the sample, independent of gravity). The distinction is largely academic, but it is real.

    I do not doubt her knowledge, I have just never heard of this distinction. Is there truth to this?

    Please don't misunderstand my question: I have no doubt whatever about the difference between weight and mass, my doubt is about the unit of measurement being pounds vs. kg.
     
    Last edited: Feb 27, 2007
  2. jcsd
  3. cristo

    cristo 8,412
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    Are you asking if there is a difference between mass and weight? If so, then yes; your weight is a force, and depends on the gravitational field in which you are being weighed, whereas your mass is an invariant quantity (i.e. doesn't depend on where it is being measured)

    Note that, here, the pound being used is the "pound-force." There is also, to confuse matters enormously, a unit of mass called the pound (or "pound-mass").

    Edit: Your last line wasn't there when I replied! Still, the confusion is probably arising due to there being two different quantities with the name "pound." In my experience, a pound is a unit of mass-- but then I was answering a homework question which used the pound as a unit of force (or weight). It was only then that I found out about this "pound-force," since I've only ever used the pound as being the imperial unit of mass!
     
    Last edited: Feb 27, 2007
  4. Yes, you are correct...sort of. Some governments have officially declared "pound" as a unit of mass, including the US and UK. Pound-force is the US and UK's official unit for force or weight due to gravity.

    Anyone who frequents the forum knows DaveC know what mass and weight are.
     
  5. cristo

    cristo 8,412
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    :blushing: Sorry, Dave!
     
  6. DaveC426913

    DaveC426913 16,439
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    I ... think I'm flattered... :confused:
     

  7. Like I'm going to listen to the government:biggrin:. Officially for me, a pound is a unit of weight, just as a newton or a dyne is a unit of weight (or force). Grams and kilograms are considered a unit of mass.

    I often like when people say that 1 pound is equal to 454 grams and vise versa. Of course this is only true on the surface of the earth but anyway...
     
  8. Integral

    Integral 7,351
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    The (old) official unit of mass in the imperial system is/was the Slug. Pounds are units of force. So yes, there is a real difference between pounds and Kilograms.
     
  9. DaveC426913

    DaveC426913 16,439
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    So, what is a pound equal to at the top of Mt. Everest, or on the Moon?
     
  10. HallsofIvy

    HallsofIvy 40,933
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    When did this happen? Back when I was in school (fighting off the dinosaurs with my abacus) "pound" was a unit of weight and so force.
     
  11. HallsofIvy

    HallsofIvy 40,933
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    Assuming that we are now talking about "pound" as a measure of mass, one pound, of course!

    If, on the other hand, we are talking about "pound" as a measure of weight (force) (the "real" definition to us geezers), its equivalence in grams (a measure of force) would be
    1) 454 grams times (acceleration due to gravity at the top of Mt. Everest/acceleration due to gravity as sea level)

    2) 454 grams times (acceleration due to gravity on the moon/acceleration due to gravity as sea level on the earthj)
     
    Last edited: Feb 28, 2007
  12. Metric system rules!!!

    :biggrin:
     
  13. FredGarvin

    FredGarvin 5,087
    Science Advisor

    Just to throw gas on the fire, there is a kg-force unit. I rarely see it, but it is around.

    I have had to work with a lot of old calculations from quite a few years ago. It gets extremely confusing when the Lb unit is thrown in for both mass and force. I always add the modifier Lbm or Lbf. It was a definite shortcut for older types because they were interchangeable for 99% of the calculations you faced. Sometimes I will convert a problem over to metric just to make sure I haven't missed anything.

    The other maddening part is that there may be an accepted mass unit (slug) but no one seems to stick to it. Not, at least, in papers and such that I read.
     
  14. Well on the moon anyway 1 pound would be equal to 2.7 kilograms.
     
  15. pound
    –noun, plural pounds, (collectively) pound.
    1. a unit of weight and of mass, varying in different periods and countries.
     
  16. DaveC426913

    DaveC426913 16,439
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    Why? What is that formula saying?
     
  17. Integral

    Integral 7,351
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    For the most part standard dictionary definitions are held in pretty low regard when discussing physics terminology. You need to look in a physics text to get the formal definitions.
     
  18. D H

    Staff: Mentor

    Pounds are units of mass, force, and money. At one point in time, a one pound mass of silver subject to one standard gravity exerted a force of one pound and was worth one pound sterling. A nice, consistent set of units, no?

    In the US, the term pound without any qualifier refers to the avoirdupois pound, a unit of mass. One pound (avoirdupois) is 0.45359237 kilograms, exactly (whether one is on the surface of the Moon or the surface of the Earth). There are other pounds that are also units of mass. For example, in the apothecaries' system, there are 20 grains per scruple, 3 scruples per dram, 8 drams per ounce, and 12 ounces per pound. This system makes much more sense than the avoirdupois system, which has 27.34375 drams per grain, 16 grains per ounce, and 16 ounces per pound. :yuck: (There is no tongue-in cheek smiley, so I used yuck instead.)

    A pound is also a unit of force. To avoid confusion, the pound-force is abbreviated lbf. One pound (force) is 4.4482216152605 Newtons, exactly (whether one is on the surface of the Moon or the surface of the Earth). Of course, one pound-force is 16 ounces-fource.

    Confused? Read again after taking a few drams of whiskey for mental clarity. (Note that drams are also an English unit of volume, used specifically for measuring Scottish whiskey).
     
  19. Do you not get what i'm saying:confused:?? On the moon 2.7 kilograms is equal to one pound. All that formula is is newtons law of gravitation. I would think you would know that formula but to make sure you do i'll explain. The formula is-
    G*m*m/r^2=f
    G = gravitational constant (6.67e-11)
    m = mass in kg
    r = distance in meters.
    f=force in newtons

    All that link is saying is that a mass of 2.7 kilograms on the moon is equal to a pound.
     
  20. D H

    Staff: Mentor

    Not quite right. Read the results. It says 0.987 pounds force, not pounds. Pounds are units of mass, not weight.

    The Google calculator http://www.google.com/search?num=100&hl=en&safe=off&rls=com.microsoft%3Aen-us%3AIE-SearchBox&rlz=1I7GGLR&q=2.7+kilograms+in+pounds&btnG=Search knows the difference between pounds (unit of mass) and pounds force (unit of force). 2.7 kilograms is 5.95 pounds, whether you are on the Earth, the Moon, or Jupiter.
     
  21. D H

    Staff: Mentor

    The National Institute of Standards and Technology are the official US standard bearers regarding the definition of English system as implemented in the US. The pound (avoirdupois) has been defined as 0.45359237 kilograms, exactly, since 1959. This is the International Pound, the same pound used by other English-speaking nations. See http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf for details.
     
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