Mass units
SI Definition
Although it has a kilo-prefix the kilogram (Kg) is the SI Unit for mass. That is to say the gram is not the base unit, and the Kg simply 1000 of them, the Kg is the base unit and the g is 11000th of that (see SI prefixes also)
Currently it's defined by the weight of a "reference object" - that is some thing we[Note 1] made with careful choice of shape and materials which we simply call "1 kilogram" even if it changes (and it does) and irrespective of what the scale we "weigh" (cough) it on. However there are plans to define it by a number of atoms (cf: Avogadro's constant) and make a sphere with pretty close to that number I believe.
That's a whole topic of itself though.
Historically
Historically the Kg was related to volume via "water"[Note 2] as exactly 1L of water (@ 4∘ C - when water is its densest)
In turn the Litre is 1000 cm3
A meter is of course 1000 cm, so an m3 volume is 100×100×100=1,000,000 cm3
- Thus 1 cm3 = 11,000,000 m3
So we see that 1L = 1000 cm3 = 11,000 m3[Note 3]
So one way of defining the Kg is by having the meter defined, however I imagine getting reliably the same amount (volume) of water, accurately at a temperature is quite difficult. For example surface tension and what it forms on contact with the container's walls will be difficult. Hence the search for alternatives.
Conversions to other units
| Unit | Multiplier (to get Kgs) |
Comment |
|---|---|---|
| Pound (lb) | 4.535 923 7×10−1 [Kglb] (or: 0.453 592 37) |
"Avoirdupois pound"[1] - not other forms (there are a lot of different pounds) |
Notes
- Jump up ↑ Humans
- Jump up ↑ Then water at a specific temperature, probably also had to be distilled I'd imagine as that'd cause changes too...
- Jump up ↑ = 1,000×11,000,000
References
- Jump up ↑ https://www.nist.gov/pml/nist-guide-si-footnotes#f22 (as of 0653 (UK TIME) on the 10th of March 2019), from page https://www.nist.gov/pml/nist-guide-si-appendix-b9-factors-units-listed-kind-quantity-or-field-science. "avoirdupois pound"
