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  1.  (6376.21)
    Planck units, bitches. Planck ain't nuthin' to fuck wit.
  2.  (6376.22)
    @BrianMowrey Thank you!

    Another neat thing about this, to think about : if we're defining a kilogram as a number of atoms, that's how much they weigh in certain relation to earth's gravity, but it's still a certain number of atoms. What happens when you take that same number off-planet? Is a kilogram still based on the number, or what that number represents in certain circumstances?
    • CommentAuthorZJVavrek
    • CommentTimeJul 17th 2009
     (6376.23)
    A kilogram is not a measure of weight. It is a measurement of mass, so the local gravitational field doesn't really change it. Newtons (a measurement of force, apparently) are used to measure weight.
  3.  (6376.24)
    Another neat thing about this, to think about : if we're defining a kilogram as a number of atoms, that's how much they weigh in certain relation to earth's gravity, but it's still a certain number of atoms. What happens when you take that same number off-planet? Is a kilogram still based on the number, or what that number represents in certain circumstances?

    Which is kind of the point of the Avogadro project (and other atom-counting definitions). If they can define a kilogram as X number of atoms of silicon, then a kilogram would be defined as the mass of those X number of silicon atoms under any gravitational force. If a kilogram weighs 2.2046 pounds on Earth, we can also know what that kilogram weighs where gravity is more or less than Earth. (We already know that now, I know, but atom-counting definitions take away the weight-based definition, and create one that is universal regardless of the force of gravity.)
    • CommentAuthorFan
    • CommentTimeJul 17th 2009
     (6376.25)
    > If they can define a kilogram as X number of atoms of silicon

    All those atoms must be of a single, specific isotope of silicon, incidentally.
  4.  (6376.26)
    Which is kind of the point of the Avogadro project (and other atom-counting definitions). If they can define a kilogram as X number of atoms of silicon, then a kilogram would be defined as the mass of those X number of silicon atoms under any gravitational force. If a kilogram weighs 2.2046 pounds on Earth, we can also know what that kilogram weighs where gravity is more or less than Earth. (We already know that now, I know, but atom-counting definitions take away the weight-based definition, and create one that is universal regardless of the force of gravity.)
    You should really look up Planck units a.k.a God's units (huhuh), since they're based on universal constants (Speed of light in vacuum, Gravitational constant, Planck's constant, Coulomb force constant, Boltzmann constant) and not on some random shit us hairless monkeys came up with.
    Wiki on Planck units
    •  
      CommentAuthorjohnjones
    • CommentTimeJul 17th 2009
     (6376.27)
    The kilogram lags behind the other six SI base units (meter, mole, second, kelvin, ampere, candela) in being assigned a definition based on a fundamental physical property.


    The problem is that of mass and weight. You can't really define weight in fundamental terms because it can change. Weight is the force gravity exerts on an object and derives from mass. If a meteorite hits Earth, everything weight will increase very, very slightly because Earth's mass (and gravitational pull) has increased due to the mass of said meteor. Likewise when we send satellites into orbit, Earth's mass decreases by the mass of those various satellites.
  5.  (6376.28)
    Aye, but the mass of the kilogram still equals the mass (the exact accumulated total physical presense of protons and electrons and vibrating massless string whatevers) of the International Prototype, whether or not asteroids hit or Galactus stops by and uses the Pacific Ocean to go Number 2 after a rough night.

    Furthermore, all acceptably-precise balance scales would still work as well as they always have in any condition of stable gravity, because a balance scale already calculates mass, not weight. You can sell crack just fine on the moon.

    The kilogram's current definition is a perfectly precise definition of mass.
  6.  (6376.29)
    So when Galactus takes a wade in the Atlantic, all the boats go up with the rising tide, making the difference moot.
  7.  (6376.30)
    The problem is that of mass and weight. You can't really define weight in fundamental terms because it can change. Weight is the force gravity exerts on an object and derives from mass. If a meteorite hits Earth, everything weight will increase very, very slightly because Earth's mass (and gravitational pull) has increased due to the mass of said meteor. Likewise when we send satellites into orbit, Earth's mass decreases by the mass of those various satellites.

    You can define mass in fundamental terms if you use Planck mass.