It’s used in scientific papers all the time. I often use Mm to describe the size of features on the surface of the Sun.
It’s mostly done by writers and journalists who think their readers don’t know what a Gm is, but figure out that a million kilometers is a long distance. I think they are right too…
And some non-Metric units still survive for reasons of tradition and convenience. Using AU and Parsec to measure astronomical distances, for example. (And Angstrom, which is really annoying because you can never get the symbol to display correctly when you send papers by e-mail.)
It means other SI units are derived from the kilogram, meter and second. For example, the joule is defined as kg m[sup]2[/sup] s[sup]-2[/sup]. The famous formula e = m c[sup]2[/sup] only works if you express the mass in kg and c in m/s; then it produces a result in joule.
No, it works for any units, if you use your mass and speed units in the equation and convert the result to your energy unit of choice. It also works, without any unit changes, for any system where the unit for Energy = the unit for Mass x (the unit for speed)[sup]2[/sup]
Unless there’s a special cgs unit for speed (I was raised on MKS/SI only) it works, “unitless”, for that system too.
Of all the metric prefixes, only a handful are commonly used. That’s fine. But as the father of five I have long noted that every stinking year every stinking science class spends the entire first stinking week of the school year doing dozens of prefix conversions for all these stinking prefixes that nobody but nobody ever uses.
Even in Star Trek NG they still say “500,000 Kilometers” , instead of 500 megameters or .5 gigameters.
Maybe a kilometer is the biggest distance a human can relate to, so we use the high numbers of small measurements so we can at least form a visualization of the distance in our tiny heads and somehow relate to them.
I bet thats why all these measurements are like they are…its all about the person and their relative size. I certainly coulnt picture a “teraliter” as easily as 1 trillion litres…although even thats a bit hard to “fathom”. heh heh.
Okay, so maybe it’s only because I live in the states that I haven’t heard some of the less extreme prefixes. I thought kiloliters and such weren’t used, but maybe they’re just not used here. I guess I can understand the more extreme prefixes being used only in scientific documentation, although I’ve read plenty of scientific literature and only encounted some of them.
I think the large prefixes are becoming better known thanks to computer technology. I’m glad I can say I have 2 gigabytes of RAM instead of 2,097,152 kilobytes. I wish I could say I own 5 square megameters of land though and people understand me. Hell, I’d settle for owning 5 square megameters of land even if people didn’t understand me.
The kilogram isn’t defined as the mass of one litre of water; it’s defined as the mass of the prototype kilogram, a block of platinum being preserved in the International Bureau of Weights and Measures in Paris.
The metre used to be defined the same way - there was (or is) a prototype metre, a stick of platinum with two marks, the distance between which is defined as the metre. Nowadays, the metre is defined differently, namely as the distance light travels at a specified fraction of a second (the second is defined using the frequency of a certain radiation emitted by caesium atoms.
There are plans to drop the traditional definition of the kilogram and replace it with a different approach, probably using a specified number of atoms of a specified element. But as to 2006, a kilogram is simply the mass of the prototype kilogram in Paris. I’m pretty sure its mass was modeled intentionally to match that of the litre of water, though; but since the density of water depends on pressure and temperature, the equation 1 litre of water = 1 kilogram cannot be generalized.
But this question of definition doesn’t affect the basic idea behind the metric system: On one hand, you can use SI units in any formula,. and the result will be in SI units as well. Take the formula for kinetic energy (E = .5 mv²), for example, and you see what advantage you get using kilograms and metres per second instead of pounds and feet per second. On the other hand, you have the system of prefixes; it’s true that many of them aren’t too common (although you do hear things like decilitres and dekagrams in countries that have been metric for a long time), but 150 million kilometres don’t cease to be 150 million kilometres just because you don’t express this distance as 150 gigametres.
I think you are correct in saying that the kilogram is the base unit because of this standard cylinder that is kept in Paris and I suppose they could have made a standard gram but they didn’t. Perhaps a kilogram sized mass is easier to work with. This cylinder is or was removed periodically to calibrate standards around the world. A gram mass would be too small to work with and create too much uncertainty when trying to calibrate other masses as it is only 1/1000 the size of the kilogram.
Besides, a gram just looks wimpy in a cylinder.
BobLibDem touched on this but it’s such an important idea that I’m going to run with it. The reason most people use the metric system in the USA is to perform scientific calculations. It is in the best interest of the people doing the calculations to have them be as simple as humanly possible. To this end, having simple units is an enormous boon. I would actually be happy if the metric system had no prefixes and everything was expressed in terms of fundamental units only.
Imagine you are trying to find the kinetic energy, in joules, of something (KE = 0.5 mv[SUP]2[/SUP]) and some goon gives you the mass in teragrams and its speed in meters per kilosecond. He have essentially given you about five new chances to make a mistake somewhere, and that is the simplest example I could come up with. If you have to carry multiple numbers through multiple and complicated equations and worry about converting unorthodox units, you’re in for a nightmare. Once you’re done with the calculations, fine, go ahead and express the answer in teragrams-megaseconds-squared, but the purpose of the metric system is to be useful. Sticking to standard prefixes is not a “waste.” If your boss told you that your lunch break is 3 kiloseconds long or that you have a meeting in 0.125 fortnights, I don’t think anyone would be grateful that he’s somehow taking full advantage of the wide variety of units arrayed before him like a cornu copiae of textures and flavors. Units are meant to convey specific information and you can really hide their meaning by using weird ones.
I hope I’m not being too harsh. Units and I didn’t get along too well in college. Don’t even get me started on CGS vs. MKS.
From http://en.wikipedia.org/wiki/Meter
The metre (US spelling: meter; symbol: m) is a measure of length, approximately equal to 3.28 feet. It is the basic unit of length in the metric system and in the International System of Units (SI), used around the world for general and scientific purposes. Historically, the metre was defined by the French Academy of Sciences as 1/10,000,000 of the distance from the equator to the north pole through Paris. Now, it is defined by the International Bureau of Weights and Measures as the distance travelled by light in absolute vacuum in 1/299,792,458 of a second. I believe the kilogram is the only measurement still defined by a physical object.
I don’t see how this is any different than in the imperial system. American science texts often give velocities in feet/second or miles/hour. You use a simple conversion factor to go from one to the other. The only difference with the metric system is that if you know your prefixes, you can do the conversions in your head.
Except for time damnit, we should have listened to the French!!
The Angstrom, in my opinion, is kinda weird since it’s E-10 meters. I guess it’s slightly more convenient than saying 0.1 nm, since you can work in integers instead. You don’t really see the micron used these days, though personally I think it’s easier than saying micrometer (and really means the exact same thing.)
In chemistry I often work with prefixed units like mol and M. The compounds I’m working with are heavy enough (generally with a molecular weight of 400+) that on most scales I’m dealing with millimols and millimolar solutions. Of course that’s even more common in biochemistry. I was doing some titrations the other day and my stock solution was 5 mM and the volume I was using worked out to my having μmol of my compound present.
The trend is to making our reference standards as universal as possible. As Schnitte alluded to, the second is defined as “the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This definition refers to a caesium atom at rest at a temperature of 0 K.” The meter is defined as “the distance travelled by light in absolute vacuum in 1/299,792,458 of a second.”
Is there any elegant way to define the kilogram in terms of moles?
But that’s quite an advantage. I don’t want to argue in a “we Europeans do it better” way, nor do I want to hijack this thread to the dull argumentation on whether America should switch to metric. But the metric system allows you to do all the conversions in your head because every prefix involves a multiple of 10, which is easy to compute; the imperial system operates with factors such as 12 and 16, for which most people will need a calculator. Of course pocket calculators are small and cheap enough today to have them at hand whenever necessary; yet I think a system that does totally without using calculators is still preferable. Besides, you don’t only need a calculator but also a table of conversion factors - not all calculators have defined functions to convert units into each other. The only conversion factor I ever had to memorize during physics classes was 3.6 (to convert meters per second into kilometres per hour).
This is one of the few SI measurements that goes into negative numbers as well. Someone with a beauty of -3 millihelens is ugly enough to pull 3 ships to shore.
Until the invention of the ship’s thruster, this was very useful in docking maneuvers.
Well yes, I was just saying if you use the standard SI units, you get the answer in a standard SI unit. And while this formula is fairly well in all units, there are others that are much easier to use in one than the other, to the point that textbooks provide different versions for SI and cgs (e.g. Maxwell’s equations).
Sometimes I wonder why people try to criticize/nitpick other posts rather than guessing what they meant to say and try to provide clarification…
Planetary rings are on the order millions of meters in radius, but planetary people, for some strange reason, don’t like Mm. Instead, they use, I kid you not, kkm. Kilokilometers. A thousand thousand meters.
If I had a time machine, I’d go back and find the first person to use that notation and kick them in the kiester.
It has to do with a variation of Murphy’s law: If there is more than one way to interpret a statement, and one or more of those interpretations are wrong, then someone will read them that way, even if it’s fairly obvious that’s not what author meant.
Besides, what you actually wrote was that E=mc[sup]2[/sup] only works for SI units, which is wrong for all interpretations I can think of.
