# Unit System
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A **unit system** is composed of the variants upon one standard scientific unit of measurement. The metric system, for example, contains a basic unit, meters (m), as well as numerous variants including kilometers (km) and centimeters (cm). Other useful **unit systems** include mass (g, kg, mg), volume (L, mL), and time (sec, min, hr).

Okay now we're going to talk about some of the units and prefixes that we use in Science and why do we do this? Well kind of again get rid of all the zeros before or after the number that we really care about. So let's look at 2 of them that are less common, the units for 10 is deca but 10 is not a terribly hard number to write 17 or 13 we don't use that very often. The unit for a hundred is hecto again not very hard to remember a 172 or to write 172. But occasionally you'll something like hecto acres in terms of land of a farm or something like that. Much more common is the term or the unit for a thousand and a thousand is a kilo. And we abbreviate that lower case k so you probably no doubt heard of a kilometer or a kilogram of a mass, so those are quite common in Science units.

Another one that's important to know is a million and again you probably have heard this with your computer, your storage the megabytes so in mega is a million and we abbreviate that big M. Getting even bigger you know how computers and storage whether it's your computer or phone you need more and more memory in there right, so no more megabytes. Now we want to go to one billion of a unit and a billion of a unit is a giga and we capitalize that big G okay so instead of having to write those 9 zeros after the 1 we can just write giga, gigabytes, gigagrams okay.

Getting even bigger than that, is one to the twelfth which is a trillion. One trillion of something is a tera and big T so my phone doesn't have terabyte storage yet but hopefully in the next year or two we'll have a technology that gets that big, so terabytes one trillion then again scientific notation for a trillion is 1 times 10 to the 12th which would be a 1 with 12 zeros after it that's a lot of zeros that you don't have to write so you can just write a tera.

Getting smaller okay if we go down in the lower direction one tenth of something is a deci which we'll abbreviate little d okay so decimal right next to the decimal is a deci unit, a decigram or a deciliter. Okay one, one hundredth of a unit this is much more common you've probably heard of one, one hundredth of a dollar is a cent so a centi and centimeter is a common unit you use in Science for measurement. One, one thousandth of a unit again Scientific notation for that should be 10 to the negative 3 write 3 zeros to the right of the decimal point. One, one thousand would be a milli, I'm sorry this is one is a little case c, this milli should be lower case m and in no doubt you've heard of Aspirin or Tylenol in milligrams of strength, the active ingredient they tend to measure medicines in milligrams a quite common unit.

Okay one millionth of a unit again Scientific notation for a million is 1 to the negative 6 and one millionth of a unit is a micron, now we have an m for milli so we can't use a lower case m for micron so we're going to go back to Greek and we're going to use the Greek m which kind of looks like curse of u and you'll see this in Biology a lot because when we measure diameters of cells, we typically measure one millionth of a meter for the size of a red blood cell and that unit is microns or micrometers. Very common unit for studying small things in Biology.

Getting even smaller than a millionth of a unit is one billionth of a unit or 1 times 10 to the negative 9 and the unit we use for that is a nano lower case n to abbreviate so nanometers, nanograms one billionth of a gram. And even smaller than that is one trillionth of a unit now one trillionth we use pico which is lower case p and that is again one trillionth of a unit or 1 times 10 to the negative of 12, if we don't want to write all these 12 zeros after the decimal we can just write pico, picograms, picoliters etc. So these are all the common prefixes that we use in Science to avoid all those zeros on either side of the decimal and help simplify our life.

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