The adventures of a fourth grade teacher in East Central Illinois.

Learning Metric Prefixes

We started my favourite mini-unit in math today: the metric system. I make no secret of my bias toward the metric system. It is neat and orderly. If you  know the basic units and you know your prefixes, you can figure out how to convert from one unit to another by simply multiplying or dividing by tens. This makes so much more sense to me that the US Customary System, which has neither rhyme nor reason to its system of conversion. (If it were not for a handy magnet on my refrigerator, I would never be able to keep track of the number of tablespoons in a given fraction of a cup!)

While some of my students have had run-ins with the metric system before, today was, for most, their first serious introduction. We started with length and then will proceed through volume and mass before wrapping up. (Remember, this is a mini-unit!) I started the unit as I typically do: I held up a meter stick (sometimes written meterstick) and asked the class if anyone could identify it. One student suggested it was a yardstick, but recanted when I pointed out that there were 39 3/8 inches (approximately) and a yard is just 39 inches. Someone else recognised it as a meter stick and then we talked about what you would measure with meters.

It became quickly apparent that the meter is simply too large to measure most everyday objects, which led to a discussion of the smaller units of the meter, such as the decimeter, centimeter, and millimeter. (One student also mentioned the nanometer, but there are few reasons to measure with that length in a fourth grade classroom!) Instead of just telling the students what these other units of measure equated, I wanted to see if they could make the connections between other words with similar prefixes. We talked about a penny is worth one cent, or one hundredth of a dollar, and how a century is one hundred years. By looking at the meter stick, they were able to determine that a centimeter is one hundredth of a meter. Then we talked about millimeters and we were able to reflect that a thousand years is known as a millenium, so a millimeter is on thousandths of a meter. One student then asked why certain arthropods are called centipedes and millipedes. I explained that while centipedes do not have one hundred legs and millipedes do not have one thousand legs, there was most likely someone at some point who saw them and described them as such. Alternatively, the centi- and milli- prefixes may simply be used to describe “many” and “many more.”

Unfortunately, some of the other metric prefixes, such as hecto- for hundred and kilo- for thousand are not as easy to remember, as these prefixes do not get used much outside the metric system. The only word I could think of, hectare, is actually an obsolete metric term! When searching for the etymology of kilo- I learned that it was adopted by French for the metric system from the Greek khilioi, but there is not much known beyond that. On the other hand, it is pretty easy to remember deca- for ten, as long as one can remember that deca- is ten and deci- is tenth.

We won’t be getting much into all of the other prefixes used in the metric system, but I hope that my students will work on remembering these basic ones! We will review them and practice each day as part of this mini-unit and we will have a poster in the room to help us remember the relationships between these prefixes and their decimal values.


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