201–210 of 589 posts
See, the US wasn't being stubborn in not adopting metric, it was just waiting patiently for it to progress from beta to 1.0. :) (Fun fact: all the US customary units have been officially defined in terms of their metric counterparts since 1893: https://www.nist.gov/sites/default/files/documents/pml/wmd/m... )
(the opposite of https://xkcd.com/2073)
Has a "metric" version of time ever been proposed? Seems odd to me that we're "ok" with divisions of 12 for time but not length. I guess for science seconds are basically the only measure of time that usually matters?
The French Revolution attempted a decimal calendar alongside their switch to metric. It's a fascinating artifact: https://en.wikipedia.org/wiki/French_Republican_calendar Calendars are hard because you've got multiple inconveniently disparate sources of information you want to unite: universal constants on the one hand (such as the second being tied now to atomic vibration) and more "local" concerns such as the orbit…
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24 and 60 do make sense because they are highly composite numbers: https://en.wikipedia.org/wiki/Highly_composite_number
12 and 60 are superior highly composite numbers¹ — as is 360, the number of degrees in a circle and the number of days in a year (with a little engineering work TBD). Speaking of circles, SI hasn't fixed Hz vs rad, have they? ¹ https://en.wikipedia.org/wiki/Superior_highly_composite_numb...
I don't think so.
Anyone who hasn't heard of the Hz/radian issue should see the "Hz" definition in the units definition file shipped along with Frink - if you like finding interesting rants in unexpected places, it's delightful:
Has a "metric" version of time ever been proposed? Seems odd to me that we're "ok" with divisions of 12 for time but not length. I guess for science seconds are basically the only measure of time that usually matters?
See, the US wasn't being stubborn in not adopting metric, it was just waiting patiently for it to progress from beta to 1.0. :) (Fun fact: all the US customary units have been officially defined in terms of their metric counterparts since 1893: https://www.nist.gov/sites/default/files/documents/pml/wmd/m... )
So it's fair to rib my American friends with the fact that Imperial is implemented using metric?
Note: Americans hate it when I do this.
See, the US wasn't being stubborn in not adopting metric, it was just waiting patiently for it to progress from beta to 1.0. :) (Fun fact: all the US customary units have been officially defined in terms of their metric counterparts since 1893: https://www.nist.gov/sites/default/files/documents/pml/wmd/m... )
Now can we redefine 1 pound to equal 1 kg? (the opposite of https://xkcd.com/2073 )
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You could just define the second to be a 1/100,000 of a day and go from there. Aside from screwing with literally everyones perception of time, it could work.
But which day? Such a second would probably work for everyday purposes (just like to most of us a second is 1/86400 of a day) but it would be woefully inadequate as a universal fundamental unit of time. And defining the second in terms of something like "the length of the solar day at Greenwich meridian on Jan 1 2020" prevents anyone from ever reproducing the exact value based on their own measurements, which is one…
[0]: https://www.nasa.gov/topics/earth/features/japanquake/earth2...
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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 aspect…
I think the GP is making the claim that Avogadro's constant should be 1 (effectively eliminating it), not that there's no need to count molecules.
You need to convert from mass to numbers of molecules, which means you need to divide it by the mass of a molecule. The mass of a molecule is determined by the sum of the weights of each of the atoms, themselves the weights of their constituent nucleons [1]. If you fix the weight of a nucleon to be 1 (that is, we measure in daltons), then computing the weight of a molecule such as glucose (aka C₆H₁₂O₆) in daltons is a trivial formula. All you need is a periodic table that lists atomic weights, which is every copy you find a chemist using. It's worth noting that the resulting molecular weights are going to be independent of whatever measuring system you want to use [2], whether it be grams, ounces, alien flits, what have you.
Now you need to convert the mass of your substance into a count of "stuff-loads" of molecules. The simplest and most idiotic thing to do is to define a "stuff-load" to be the amount of molecules in a unit mass if it weighs 1 dalton--in other words, you make this formula be exactly one. In SI, the unit mass for this equation is grams and the "stuff-load" is the mole. If we were using US ounces as the unit mass, we'd define an ounce-mole and use that instead of SI moles.
Put another way: we define a mole such that the constant in the computation of moles from molecular weight and mass is exactly 1. Avogrado's constant itself is merely the inverse of the mass of a nucleon when expressed in grams.
[1] Okay, there's a lot more that goes on into the computation of mass. In terms of the mathematical error, though, other sources of error (e.g., wrong isotopic ratio) are going to matter before these come up.
[2] Up to the slight adjustment (about ±1%) of what you consider the weight of a nucleon to actually be.
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My understanding is that the cause is error propagation. We can measure certain kind of masses really accurately in unified atomic mass unit. With better relative precision than the Avogadro constant. But if we used kilograms to write down these quantities then they would carry the error of the Avogadro constant needlessly. Edit: I think your question boils down to "Why do we have two separate units for mass, u and k…
Right, I get that, and if anything I feel less dumb now that I've asked it publicly and people seem to think that it's a non-dumb question, but when talking about fundamental constants I understand there is a practical nature to it all, but just as we have a nano-meter and a light-year I figured we'd have the same for mass. Why do I need both kg and u as fundamental constants? I believe the comments here have answere…
That's not a whole answer, but it may be helpful for your thinking.