Boeing and metrcication question

Ah, you are confusing things! SI-units are the smallest possible set of units to do all the calculations. Metric units are based on the SI-units. They do have a few "shortcuts" like the liter or bar (= kPa). F.e., the liter is clearly derived from SI (1 dm^3). But nothing like kgm, as kg is already mass. The micron is not a new unit, it is just an lazy abbrev. of micrometer (µm).

Some old non-SI-non-metric units seem to be more persistent. Like calorie or HP. Calorie only for nutrition, HP almost only for cars.

And coming back to SI and liter. As soon as you do make math with it, the liter gets unhandy. So you'll convert it as soon as you write down its value. Example, that also shows the advantage of the factors-of-ten-thing: Volume = 4.2 l = 4.2 dm^3 = 4.2 * 10^-3 m^3

And an example (verbose) to do math with the units: If I want to know the height of water poured onto 1m^2 (with the values from above)

height = Volume / area = 4.2 * 10^-3 m^3 / 1 m^2 Units: m^3 / m^2 = m^3 * m^-2 = m Result: 4.2 * 10^-3 m = 4.2 mm

That's elegant! No pocket calculator involved.

Nick

Yes, we've established that.

Uh, it was the other way around, Nick. Metric units existed long before the SI.

Well, this is a good example of what I've been talking about. The definition of a kilogram is meaningless to non-scientists who have to use the quantity in everyday life. The meaningful quantity is the force that's exerted on a kilogram of mass by gravity. That's the one they can feel, that's the one that's consequential in their lives and work, and that's the one everyone (except scientists) relates to.

Thus, the kgm remains the *sensible* unit. The "proper" definition of the kilogram is an abstraction that is essentially meaningless if you don't have a laboratory.

In the US, and possibly in the UK, we avoid the use of "micrometer" because of possible ambiguity. For example, the sentence "We measured the length of the screw to the accuracy of a micrometer."

True enough.

Yes, it's elegant, and very useful if you have to do such calculations and you know what you're doing. Most people don't. Thus, they avoid those multi-dimensional units and use the single-dimensional units that the SI doesn't like.

I'm tempted to summarize what we've been saying but it would be too involved. The point is that the SI has been a big disappointment to those who thought it would take over completely for other measuring systems. There are several reasons it hasn't, the primary ones being the clumsiness of multi-dimensional units when all you're doing is adding and subtracting them (the vast majority of everyday calculations); the fact that the multi-dimensional units tend not to have an analogy that can be experienced by the senses; and the fact that for dimensional and volumetric measurement, experience has shown that it rarely matters what base you use for units.

Thus, the SI remains the province of most scientists and many engineers, while the older metric and the inch/Imperial system cruise merrily along, with no apparent hardship for anyone except those SI advocates who can't understand why the rest of the world isn't so clear-headed and sensible as they are. d8-)

-- Ed Huntress

This is **pure** **bull-shit**. Repeating it after being told that it is wrong only explains the other nonsense you told about SI and metric.

kg *IS* *MASS*!

EOD, Nick

Oop, sorry about that. I should have said "kilogram-force" rather than "kgm."

-- Ed Huntress

We overlapped in messages. I said, "sorry, I meant kilogram-force."

-- Ed Huntress

kg==single dimensioned unit m==single dimensioned unit s==single dimensioned unit

Just about all of the rest (Ampere, Tesla, Newton, Stokes, Joule etc. ad infinitum :) are multi-dimensional.

Note that Nick has been just about the only one to correctly capitalise Calorie to indicate the kilo calorie as used in food. Why aren't McDogBurgers measured in BTU's? :-)

Mark Rand RTFM

Jeez, not *all* of them are multi-dimensional. Just enough to thicken the whole process of using them in everyday measurements.

A lot of multi-dimensional SI units are the consequence of the system's minimalist approach: when you use fewer base units, more of the derived units have to combine more than one base unit.

But Nick also is the one who leans on the fact that the calorie is not a legal SI unit. d8-)

-- Ed Huntress

Have you ever tried to heat your house with them?