Metric System of Measurement
(Correctly called "SI")
The metric system is a system of measuring. It has three main units:
|m||the meter for length|
|kg||the kilogram for mass|
|s||the second for time|
we can measure nearly everything in the world!
|The length of this guitar
is about 1 meter:
|When unfolded this ruler
measures 2 meters:
This gold bar has a
A dictionary also has a
mass of about 1 kilogram.
1 second is about as long as it takes to say "one thousand and one"
Larger or Smaller
But what if we want to talk about really big or really small things?
Answer: we can use Metric Number Prefixes
- like "kilo" (a thousand)
- and "milli" (one thousandth)
- and so on
Example: something that is 1,000 meters is a "kilometer"
Something that is one thousandth of a second is a "millisecond"
In fact the kilogram already uses this method, because it is a thousand grams, a kilogram.
So one thousandth (1/1000) of a kilogram is simply a "gram"
Here is a quick summary of the special prefixes:
Example: A million liters would be called a megaliter and abbreviated ML
Example: A thousandth of a second would be called a millisecond and abbreviated ms
Making Other Units
You can also combine the meter, kilogram and second to make new Units of Measurement!
Speed is how far something moves over a period of time
So it can be measured in meters per second
It means: How many meters does something travel in one second
You could write it as meters/second, or simply m/s
Here are a few common units that are based on the meter, kilogram and second:
Area is length by length, so the basic unit of area is a square that is 1 meter on each side.
The Unit is meters × meters, which is written m2 (square meters).
Volume is length by length by length, so the basic unit of volume is a cube that is 1 meter on each side.
The Unit is meters × meters × meters, which is written
m3 (cubic meters).
So, a cube that is 1 meter on each side is a cubic meter (m3) ...
... and that is also equal to 1,000 liters.
1 m3 = 1,000 Liters
Liter is abbreviated L (some people use lowercase l, but that looks too much like 1).
So a liter is actually one-thousandth of a cubic meter.
1 Liter = 1/1000 m3
Another way of thinking about a liter is:
- A box that is 0.1 meters (10 cm) on each side,
- One square meter that is millimeter thick.
An hour is 60 minutes, and a minute is 60 seconds, so an hour is:
- 60 × 60 = 3,600 seconds
A day is 24 hours so:
- 1 day = 24 × 60 × 60 = 86,400 second
Speed in meters per second (m/s)
This is a combination of two units (meters and seconds) to make a new one (m/s).
If something is traveling at 1 m/s it moves 1 meter every second.
Speed in kilometers per hour (km/h)
A bit more complicated, but a kilometer has 1,000 meters, and an hour has 3,600 seconds, so a kilometer per hour is:
- 1000 / 3600 = 1/3.6 = 0.277... m/s
How did I know to make it 1000/3600, and not 3600/1000 (the other way around)? Read how to Safely Convert From One Unit to Another.
Acceleration is how fast Speed changes.
If something accelerated from a Speed of 5 m/s (5 meter per second) to 6 m/s (6 meters per second) in just one second, it has accelerated by 1 meter per second per second!
That is two lots of "per second" and is written m/s2:
Force is usually measured in the Unit of Newtons, an important measurement in Physics and Engineering.
But a Newton is actually 1 kg · m / s2 (one kilogram-meter per second-squared).
So force is actually based on the meter, kilogram and second.
One way of looking at this is how much force it takes to make 1 kg accelerate at 1 m/s2.
But even if you don't fully understand this, it shows you that force is a combination of the three basic units.
The Metric System had its beginnings back in 1670 by a mathematician called Gabriel Mouton.
The modern version, (since 1960) is correctly called "International System of Units" or "SI" (from the French "Système International").
So you should really call it "SI", but mostly people just call it "Metric".
A few special units are also needed to complete the SI System:
- ampere for electric current,
- kelvin for temperature,
- mole for the amount of substance, and
- candela for luminous intensity
So the complete list is:
|Amount of substance||mole||mol|