Submitted by enby-millennial-613 t3_11bqbei in explainlikeimfive
I came across this by accident when I was looking up the word for "litre" in Japanese. I explanation I saw was because it's "lack of coherence" with the other base systems.
Now, I'm in Canada so of course we learn about the different SI units and how you just move the decimal to either go bigger or smaller. We obviously can do that between L and mL. You can understand now why the "lack of coherence" is a bit confusing.
What you're describing isn't "the metric system," and is commonly done in Imperial/American standard units in some fields (e.g. kPSI or MPSI for large units of pressure). Prefixes in units aren't monopolized by SI.
Liter isn't an SI unit simply because it's not the base unit of volume. Volume is inherently just built on distance measurements, and the SI system already has the meter, ergo the base unit of volume is the cubic meter.
"Lack of coherence" means it isn't derived explicitly from a core SI unit. The fact that it can be expressed in SI units without rounding doesn't matter.
This also ties to a huge misunderstanding that people have about the metric system. The strength of SI isn't remotely related to the prefix units (i.e. the "moving the decimal point over"). Instead, the core power of the metric system is that more complicated units are all derived directly from other units; so, 1 Pascal is explicitly 1 Newton spread over 1 square meter. 1 Newton is explicitly 1 kg accelerating at a rate of 1 meter per second per second. Building on those basic units is what makes the SI system as powerful as it is, as it means you don't have weird constants that you have to factor into all of your calculations (e.g. the wonderful world of pounds-force vs. pounds-mass).
A goal of the SI is to limit the number of base units to the minimum number possible. There is no SI base unit of volume because you can just use cubic meters. Similarly, there is no SI base unit for surface area because square meters already fulfil that need, even though hectares exist as a non-SI but SI-compatible unit.
Liters are a metric unit (it's based on the meter and powers of ten) but it is not an official SI unit because it is redundant.
In short, you can't take just base units and multiple them together to get a Litre.
What I mean by that is that in order for a unit of volume to be coherent with SI units it would have to be ONE meter times ONE meter times ONE meter. Not 1.5 meters, not 0.1234232 meters. Not any other amount of meters besides 1. And the same would go for any of the base units (second, kilogram, amp, kelvin, mole, and candela) and deriving a unit from them.
With a liter you going 0.1 meters times 0.1 meters times 0.1 meters.
This means that the "coherent" unit for volume would be a cubic meters. The thing is that's... a really big volume so we just don't use that day-to-day.
There are of course plenty of other derived units that are coherent. Like newtons, hertz, etc.
I'm learning a lot from this thread. I thought hectare was an SI unit lol.
To be more specific, the Liter is simply a special name for the cubic decimeter (1 tenth of a meter, cubed)
The Liter is defined by its relation to the Cubic Centimeter, which is equal to the milliliter.
Derived units still count as SI assuming they are coherent, coherence is the important thing here.
I definitely can say that you helpful people here helped me understand my misunderstanding on what was part of the metric system. I can't believe I thought SI was just to do with "prefixes" and "being divisible by 10s".
I am probably the definition of "lay person" when it comes to mathematics. It doesn't help that high school was many years ago lol.
> and is commonly done in Imperial/American standard units in some fields (e.g. kPSI or MPSI for large units of pressure)
Fields, as in scientific fields? Are there any that still commonly use imperial units at all?
> Liter isn't an SI unit simply because it's not the base unit of volume. Volume is inherently just built on distance measurements, and the SI system already has the meter, ergo the base unit of volume is the cubic meter.
Most SI units are not base units, for example the microsecond and the newton. SI just defines a list of units whose use is recommended, and they happened to decide that litre is not one of them. Other than the prefixes, they try and have just one unit for each kind of quantity. However, they include the litre in a list of non-SI units whose use is acceptable, along with the likes of minutes, degrees, hectares, and electronvolts.
>Fields, as in scientific fields? Are there any that still commonly use imperial units at all?
More industrial fields, but it's still pretty common in engineering R&D in fields where the academic, scientific, and industrial lines get blurred.
Automotive and Aerospace engineering, for example.
>Are there any that still commonly use imperial units at all?
Aerospace is still overwhelmingly Imperial, at least until you get to space. Altitudes are measured in feet, speed in knots, thrust and payload in pounds, power in horsepower (particularly for piston-cylinder engined aircraft).
A liter is a measurement of volume equal to 1000 cm^3 or .001 m^3 . It is not an SI unit because it can be derived entirely using an SI unit. There are only 7 base SI units, meter, second, kilogram, Kelvin, mole, candela, and Ampere (should be a Coulomb but I don't make the rules), if it's not made up of those it's not an SI unit. A Newton is an SI unit because it's a kg*m/s^2 , a Coulomb is an SI unit because it's an A*s
In SI there's:
m for length
m^(2) for area
m^(3) for volume.
No need for L, hectares, etc. It's glorious.
There are a lot of other metric units that aren’t SI, for example calorie, Celsius, dyne, statcoulomb.
L is a non-SI unit. SI unit for volume is not litre but cubic metre - m^(3)
Litre is however accepted for use with SI because it's more convenient to say than milli-cubic metre.
I thought Joule was the SI unit for energy rather than calorie?
A Liter is the volume of 1kg of water at STP (Standard Temperature and Pressure)
That is also true.
A cm^3 of water is one gram.
It was made to be universally transferable, everyone has water, there is a standard meter stick in France and it was updated to the length traveled by some light so it's universally reproducible. SAme with a second, it's the vibrations of some atom, but the idea is anyone can perform the same experiment anywhere and have the same precise measurement.
mtnslice t1_j9z7hdn wrote
You absolutely can move the decimal point to move between liters and millimeters. 1 L = 1000 mL, the decimal point has moved 3 places to the right. This is the same as 1 meter = 1000 millimeters. It’s just not moving the decimal point ONE place over.