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Pounds v.s Kilograms?

  1. Feb 26, 2004 #1
    A pound being a measure of weight (force)and a kg being a measure of mass - how would I convert lbs. to Kg? If F=ma, Kg would be on the right of the equation and Lb. on the left which I don't think is equal since a lb. takes into consideration the acceleration of gravity and the Kg does not. I can't seem to put together the acceleration part. If I were to set an equation of - w=mg (w=weight)using the slug as the mass in this equation (similar to the m being the Kg in F=ma)would this be going in the right direction? Pounds v.s Kilograms?
     
    Last edited by a moderator: Feb 17, 2008
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  3. Feb 26, 2004 #2

    Chi Meson

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    YOu are so correct in being bothered about this. When we measure "weight" in kilograms, we are perpetuating a misconception.

    What we mean is "the equivalent weight of this number of kilograms on the earth's surface. Since most people will not go to a place where g is any different, it really doesn't matter on a daily basis.

    To convert "kilograms to pounds" you are actually converting "the weight of so many kilograms" to pounds. THe shortcut: take the number of kilograms and multiply by 2.2. This product will be the weight (in pounds) of however many kilograms you have.
     
  4. Feb 26, 2004 #3

    russ_watters

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    This is an issue that in practice, both systems screw up. A spring-scale measures weight, and a balance measures mass. In practice, neither differentiates between the two.
     
  5. Feb 26, 2004 #4
    Thanks Chi.
    What I am interested in is how do I get to the daily conversion of 2.2 Kg/Lb.? There has to be something or a formula to account for the acceleration of gravity. Can anyone come up with the actual conversion? I have been unable to find anything of the sort.
    Hollywood
     
  6. Feb 26, 2004 #5

    Chi Meson

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    The weight in newtons: (mass) x (g)

    the conversion: one pound of force is exactly 4.448 newtons.


    take your kilograms, multiply by 9.801 m/s^2 (that's "g") then divide by 4.448 newtons/pound, et voila.

    Or, as I mentioned, keep the factor of 9.801/4.448 (that equals 2.203) to go directly from kilograms to pounds. (Remember, if you go to mars, you need to redo this)
     
  7. Feb 26, 2004 #6
    Thanks Chi Menson!!

    That will help!
     
  8. Feb 27, 2004 #7

    russ_watters

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    That's 2.2 lb/kg. And the forumla is f=ma. Make two equations and set a=a.
     
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