A gram is a man- made amount of a fundamental unit, which is mass. But mass has no actual units so the gram is simply assigned to it by us, as the meter was assigned to the fundamental unit length. Now that two completely 'made up' things cross, the cc and a gram of water, is by design when making these imaginary units in the first place and they are very handy for general calculations but still, none of them are linked to anything fundamental or effectively, "real".
There are seven fundamental units or basic units. All others are derived, compounded (such as MPH which is length divided by time, two fundamental units) or some completely man made 'measurement' that are not fundamental or relatable to anything fundamental. The fundamental units are these:
Length
Mass
Brightness (candela)
Current (ampere)
Temperature
Mole
Time
Then there are the four fundamental forces known and they are:
Electromagnetism
Gravity
The Strong force (atomic level only)
The Weak force (atomic level only)
A cubic centimeter of water is a derived unit, consisting of length X length X length. A volume of water measured in any way NOT based on fundamental units is simply a man made 'unit', created so either 1) we can get a handle on something easily- we buy gasoline in gallons, easy and handy but meaningless in the physics world or 2) some link such as one gram is one cubic centimeter of water at a temperature, which is merely man- made units designed to align volume to mass to length or similar.
The Kibble balance has allowed us to define a kilogram by comparing it directly with a fundamental force, which means that the man- made UNIT of a fundamental measurement can now be defined by another man- made UNIT of a different fundamental force. That plus the fact that we can readily only measure one of them (force, not mass) so we can now quantify mass in the physics world.
Or at least that is how it works in my corner of the universe..... I think. :-)
Brian
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I was hoping for a 3rd grade explanation of whether a gram is now based on 1/1000 of a Kilogram as measured on a Kimble balance or is it still defined by the weight of one cc of water @4 degrees Celsius. If there was an answer to that posted already, it flew right over me. If it is still based on the one cc of water formula, I don't see how we've made as much progress as we could have in this new development.
HA! No, my level of interest isn't that high.