The Seven Base Units as Defined by SI (The International System of Units, 1960)

[DonRocks]

"SI" is an abbreviated abbreviation (!) for the French words Système International d'Unités (International System of Units). It is the modern version of the Metric System (which was first introduced by the First French Republic (*) in 1799!) The United States officially sanctioned the Metric System in 1866, but remains the only industrialized country in the world that doesn't use it as their official system of measurement (think about that for a moment).

SI was first introduced in 1960, and is built entirely on only seven "base units" of measurement - the more-complicated measurements use what are known as "derived units" which are nothing more than combinations of the base units.

The Base Units

Amount of Substance: Mole

Electric Current: Ampere

Length: Metre

Luminous Intensity: Candela

Mass: Kilogram

Temperature: Kelvin

Time: Second

When I began this post, I was going to go into a great deal of depth, but just being familiar with these seven things - what they actually mean - will put you on the fast track towards having a scientific mind. How many of us know, for example, what a "candela" or a "mole" is? Isotopes for Dummies will help you with moles (not much, but sort-of like "derived units" build on "base units," so it shall be with posts in this forum).

Does McDonald's still have a Quarter-Pounder? Well, it turns out that 1 Newton is about 1/4 pound (.224809 pounds to be more precise), so you can remember this by going up to a McDonald's drive-thru window and ordering a "Newtoner with Cheese." A Newton is a derived unit (which you won't see in the list above), but I couldn't resist throwing that one in.

It's remarkable that virtually *every* measurement can be based off of these seven. Something as basic-sounding as Speed, for example, is a derived unit - not a base unit - using "metres-per-second" as its formula, thereby combining two of the base units. The Newton is a measure of Force (and Weight), and combines *three* of the base units: Mass, Length, and Time - imagine thinking of a Quarter-Pounder with Cheese in these terms - it's not difficult to imagine that certain measurements can get extremely complex.

(*) I'm sorry to get off-track, but Beethoven's 3rd Symphony (the "Eroica," or the "Heroic") was completed in 1804, and originally named "Buonaparte" after Napoléon I, aka Napoléon Bonaparte, for Bonaparte's democratic ideals in leading the First French Republic (1792-1804). But when Bonaparte declared himself Emperor (also in 1804), thus ending the First French Republic and beginning the First French Empire (1804-1815), the politically astute Beethoven didn't like it one bit, and changed the name to a generic term: The "Heroic Symphony."

Q: What happened when Napoléon went to Mount Olive?

A: Popeye got pissed.

[Al Dente]

Here's 145g travelling at 44.257 meters/second thrown by a 2.0828 meter Big Unit. A much smaller unit got in the way.

[DanielK]

The United States ... remains the only industrialized country in the world that doesn't use it as their official system of measurement 

And that's why we can't have nice things.

NASA's Mars Climate Orbiter crash and the original Hubble mirror focus problem were both metric conversion errors.