Ask someone how tall they are and they'll probably tell you in feet and inches.
- This is surprising given that we are a metric country, but it is a traditional way of describing peoples' heights that has stayed with us.
- So why did we switch over to the metric system?
The metric system started a couple hundred years ago in France.
- It was called "Le Système International d'Unités" which is why you sometimes hear it referred to as the SI system.
- It grew out of the need to have a standard system that everyone could agree on.
The original "foot" was actually defined as the size of the current king's foot! Woof! Physicists needed a standard that everyone could agree on so that results reported in one country could be understood in another.
Length
The standard unit of length is the metre.
- Notice that it is spelled "metre", not "meter".
- Many textbooks published in the United States use the "Americanized" spelling with an "er" on the end.
- Originally the distance from the North Pole to the equator divided into little pieces was used as the standard for the metre.
- Then it was switched to a platinum-iridium rod kept at 0°C at the International Bureau of Weights and Measures in Sèvres, France.
- Since even this rod could alter in size over time, we now use the distance that a beam of light will travel in 3.34 x 10-9 s.
Mass
The standard unit of mass is the kilogram.
- At one time the kilogram was actually defined as the mass of a particular amount of water.
- It is now based on a platinum-iridium cylinder kept in Sèvres, France.
Time
The standard unit of time is the second.
- When the SI system was first created, they defined the second based on how long it took the earth to spin once on its axis! 1 day = 86 400 s.
- It was felt that this was too inaccurate.
- We now use the decay rate of radioactive isotopes like cesium in atomic clocks to measure time.
For more information about atomic clocks and to get the current time you can visit the U.S. Naval Observatory's Website or the Official U.S. Time Website. Take a "Walk Through Time" at the NIST.
- The world bases time on UTC (Universal Coordinated Time).
- In Alberta we are on Mountain Time
- UTC –6:00 during Daylight Savings Time
- UTC –7:00 during the rest of the year
- In Alberta we are on Mountain Time
- If you have a shortwave radio, tune in 5000 Hz, 10 000 Hz, or 20 000 Hz to hear the time.
Derived Units
At times we will use what are called derived units. These derived units are a way of shortening up the amount of units you have to write down after an number.
You already used these in Science 10, like Joules, the unit of energy.
- From a basic formula like Ep=mgh the units for Joules should be kgm2/s2.
- Instead, we say that whole thing is equal to the Joule (J). Not only do we get to do less writing, we also get to honour some famous physicists.
- The Joule is derived from the base units of the metric system.
It's just like making something out of Lego blocks. You can arrange the Lego blocks in different ways to derive something different, but it's always made up of the base pieces.
Metric Prefixes
In order to make numbers more "friendly" looking, the metric system sometimes uses prefixes in the front of the base units.
- Measuring the distance from Edmonton to Calgary in metres would be silly. Instead we measure it in kilometres.
- Measuring the size of an atom in metres would be just as bad, so we might use picometres.
- On the back of your data sheet, you'll find a list of the common metric prefixes... notice that "kilo" is 1000 or 103.
- You don't need to memorize these since you'll always have a copy of the data sheet handy.
- Later in this unit when we study scientific notation you'll see another alternative that we have.
- Avoid mixing scientific notation with metric prefixes... it's not considered "polite."
- It is very important to always change everything into the most basic units (like metres instead of kilometres) before doing any calculations, since almost all the formulas are based on units without prefixes.