International System of Units overhauled in historic vote

Don't miss the video for the DIY Lego watt balance.

https://www.nist.gov/si-redefinition/nist-do-it-yourself-kib...

Understanding mass in molar terms is necessary to do a lot of chemistry correctly. The mole is effectively a count of the number of molecules kicking about, although the count is large enough that terms like "quadrillion" don't cut it. (One mole is about 600 sextillion molecules). Knowing how many molecules are around lets you actually compute how much stuff can react in a given chemical environment, and other aspects of chemistry end up being pretty related to molecule counts. Vanilla mass doesn't cut it since an atom of iodine weighs about 6.6 times that of fluorine but can still only react with one other molecule.

The flip side is that the molecular count is less useful to us in the everyday world. We can gauge the weight of a kilogram much more than we can gauge a septillion molecules. And if we're trying to figure out how much stuff a shelf can hold before it collapses, it's the weight that matters, not the actual molecular count. (Note for pedants: in the familiar environment of Earth's surface, mass and weight can be treated as the same quantity in most cases.)

So mass and molecular count are both very important quantities that have importance in different fields of science, and they don't have a trivial relationship to each other. Avogadro's constant and molar mass is a way to express their relationship.

The one that made it click for me as incredibly useful was Avogrado's Law (now part of the Ideal Gas Law):

> Equal volumes of all gases, at the same temperature and pressure, have the same number of molecules.

This law, for example, explains why hot air rises. Take two equal quanties of the same gas at the same pressure. They will have the same volume. Now, heat one quantity of gas. By this law, that gas will have a larger volume at the same pressure. Because it has the same amount of mass distributed over a larger volume, it must be less dense. Therefore, it rises.

It is still not apparent to me. Is a kilogram of something now it's joules x seconds or something? Does the kilogram now vary depending on what the kilogram is of? Like, is a kilogram of Iron a different mass from a kilogram of Aluminum?

>The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.

So mass is now some sort of length times a given elapsed time?

Why should there be any separation between science and daily life?

Think about it not for the benefit of the current generation who would be forced to burden the switch, but for the next generation who could reap the benefits.

I think one issue is that by growing up with US units and then using a second system for science and engineering, it can make American students studying those topics feel like they are in an unfamiliar territory and interfere with their intuition. Most will eventually overcome this through practice and become fluent in both systems, but it's still a barrier. Science becomes a Special Discipline requiring Special Language different than what your family uses at home. Like if all science were still done in French or Latin, and you needed to study those to read a paper.

Until you have met people who grew up abroad measuring their height in cm and their weight in kg, preparing food from recipes listing ingredients in grams and mL, to whom those units are completely natural for daily life and are also the same ones they use at work when designing machines and filling test tubes, it's easy to feel that US units simply "are" the units suitable for daily life, as though that were a universal truth and not just a local cultural oddity.

Not to mention the massive net costs of having suppliers worldwide making extra sets of almost identically sized components that nonetheless aren't interoperable. Screws with 3 mm and 3.175 mm diameter, etc.