3 ms·

Urgh, more discussions involving some contrived correspondence between 1 inch and 0.01m, 1 foot and 1m, and 1 mile and 1000m. There is only the metre; divide it

Urgh, more discussions involving some contrived correspondence between 1 inch and 0.01m, 1 foot and 1m, and 1 mile and 1000m. There is only the metre; divide it up however you like, compare it to whatever imperial units you like, but "centi", "kilo", etc. are just a different way of writing ÷100 and x1000. The "real" reason to use metric is that all of the constants of proportionality are defined as 1. "1m = 100cm" is trivially true, by the definition of "centi", but 1m is also equal to all of the following, which I wouldn't even want to attempt to figure out in imperial units: 1 metre 1 Joule / 1 Newton -- Applying 1N of force over a 1m distance requires 1J of energy 1 Volt * 1 Coulomb / 1 Newton -- 1 Volt is a potential difference of 1 Joule per Coulomb of charge 1 Volt * (1 Amp / 1 second) / 1 Newton -- 1 Amp of current is 1 Coulomb per second 1 Volt * 1 Amp / 1 Newton * 1 second -- (by rearranging) 1 Watt / 1 second * 1 Newton -- 1 Volt * 1 Amp gives 1 Watt of power 1 Watt / 1 second * 1 kilogram * (1 metre / (1 second * 1 second)) -- From Newton, force is mass * acceleration 1 Watt / (1 kilogram * 1 metre / 1 second) -- (by rearranging) 1 Watt * 1 second / 1 kilogram * 1 metre -- (by rearranging) (1 Joule / 1 second) * 1 second / 1 kilogram * 1 metre -- 1 Watt is 1 Joule per second And so on. We just need to get rid of anachronisms like litres (1l = 0.001m^3), defining "kilogram" as the name of a unit, and of course prevent the use of weird language-juggling like reporting the distance to Saturn as "1000 million kilometres", or measuring energy in "kilowatt hours". Genuine question: does the imperial system even have distinct units for mass and force? I tend to see ounces, pounds, stones, etc. converted to kilograms, which implies they measure mass; but pressure seems to be measured in pounds per square inch, which would be a 2D density (e.g. used as a measure of paper quality).

Armisael16 10y ago
Yes. The pound is a measure of force and the slug is a measure of mass (1 lb = 1 slug*ft/s^2). That said, a the pound-mass (lbf for pound-force, lbm for pound-mass) is probably used more in casual conversation because it has surface gravity built it.
nayuki 10y ago
> Genuine question: does the imperial system even have distinct units for mass and force? The slug is the unambiguous unit of mass in the imperial system. It is rarely heard of, and not fun to use. https://en.wikipedia.org/wiki/Slug_(mass) https://en.wikipedia.org/wiki/Slug_(mass)
dfox 10y ago
For electrical systems, kWh (and Ah) are usually more useful units than joules (resp. coulombs), but recent tendency of quoting power in units like "kWh per hour" is really absurd.
benkuykendall 10y ago
If we just needed these quantities as the inputs to an equation, sure, we should express them as 1E12 meters or (whatever x 3.6E6) Joules. But it turns out units are not only used for computing, but for communication. I am familiar with the kilometer because I know what it feels like to walk one. I am exposed to watts because I have grown up using 60W lightbulbs. Now, you could call this history and experience needless in the context of science. But in the context of my comprehension of the results of science, they are important, however "weird" they are to compute. I would much prefer for my computer to multiply by an extra conversion factor than to re-calibrate my mind to use "more efficient" units.
- chriswarbo 10y ago
  > If we just needed these quantities as the inputs to an equation, sure, we should express them as 1E12 meters or (whatever x 3.6E6) Joules. To be clear, I wasn't advocating against the use of km, cm, kW, etc.; only against the common contrivance that "the cm", "the km", etc. are some sort of independent units with remarkable power-of-ten conversion rules. As an analogy, it's like claiming that the number ½ ("one half") is, coincidentally, the same as the result of "1 ÷ 2" ("one divided by two"). In fact, there is no coincidence; the 'claim' is just describing the intended meaning of the "x over y" notation. Likewise, powers-of-ten are the intended meaning of the "kilo", "centi", etc. notation. In other words, expressing quantities in kilometres is expressing them in metres; there's no need to use, say, scientific notation to make it "more metric". My point about "1000 million kilometres" was simply that it's overly complicated (3 different factors, written in 3 different ways: one with digits, one in English and one as a prefix); why not "1 billion kilometres", "1 trillion metres" or even "1 terametre" ;) The problem with kilowatthours isn't the "kilo" or the "Watt", but the "hours"; it introduces an unnecessary factor of 3600 compared to other units. For example, a megawattsecond (AKA a megajoule) has the same order of magnitude (1kWh = 3.6 MWs)

Armisael16 10y ago

Yes. The pound is a measure of force and the slug is a measure of mass (1 lb = 1 slug*ft/s^2). That said, a the pound-mass (lbf for pound-force, lbm for pound-mass) is probably used more in casual conversation because it has surface gravity built it.

nayuki 10y ago

> Genuine question: does the imperial system even have distinct units for mass and force? The slug is the unambiguous unit of mass in the imperial system. It is rarely heard of, and not fun to use. https://en.wikipedia.org/wiki/Slug_(mass) https://en.wikipedia.org/wiki/Slug_(mass)

dfox 10y ago

For electrical systems, kWh (and Ah) are usually more useful units than joules (resp. coulombs), but recent tendency of quoting power in units like "kWh per hour" is really absurd.

benkuykendall 10y ago

If we just needed these quantities as the inputs to an equation, sure, we should express them as 1E12 meters or (whatever x 3.6E6) Joules. But it turns out units are not only used for computing, but for communication. I am familiar with the kilometer because I know what it feels like to walk one. I am exposed to watts because I have grown up using 60W lightbulbs. Now, you could call this history and experience needless in the context of science. But in the context of my comprehension of the results of science, they are important, however "weird" they are to compute. I would much prefer for my computer to multiply by an extra conversion factor than to re-calibrate my mind to use "more efficient" units.

chriswarbo 10y ago
> If we just needed these quantities as the inputs to an equation, sure, we should express them as 1E12 meters or (whatever x 3.6E6) Joules. To be clear, I wasn't advocating against the use of km, cm, kW, etc.; only against the common contrivance that "the cm", "the km", etc. are some sort of independent units with remarkable power-of-ten conversion rules. As an analogy, it's like claiming that the number ½ ("one half") is, coincidentally, the same as the result of "1 ÷ 2" ("one divided by two"). In fact, there is no coincidence; the 'claim' is just describing the intended meaning of the "x over y" notation. Likewise, powers-of-ten are the intended meaning of the "kilo", "centi", etc. notation. In other words, expressing quantities in kilometres is expressing them in metres; there's no need to use, say, scientific notation to make it "more metric". My point about "1000 million kilometres" was simply that it's overly complicated (3 different factors, written in 3 different ways: one with digits, one in English and one as a prefix); why not "1 billion kilometres", "1 trillion metres" or even "1 terametre" ;) The problem with kilowatthours isn't the "kilo" or the "Watt", but the "hours"; it introduces an unnecessary factor of 3600 compared to other units. For example, a megawattsecond (AKA a megajoule) has the same order of magnitude (1kWh = 3.6 MWs)