Do you have any suggestions for how I can help my middle school students understand and use the metric system? We struggle with this at the beginning of every year. –E., Indiana
It’s hard for U.S. students to get a handle on meters, liters, and grams when in their everyday lives they are surrounded by references to miles, feet, inches, quarts, and pounds (one exception being a two-liter bottle of soda). We can’t change what’s on television or on the internet, but we can require that students use International System (SI) units and measurements in their science activities, investigations, and reports.
The first textbook I used had a chapter devoted to the metric system, so I would dutifully “cover” it at the beginning of the year, supposedly to prepare students for future investigations. The students memorized the prefixes, and there were exercises in measuring classroom objects. The chapter had a heavy emphasis on converting units from metric to English or vice versa. What a disaster! I felt like I was teaching more arithmetic than science. Even though we practiced measuring things with metric rulers, graduated cylinders, and balances, I found that later when it came time to actually apply those skills in investigations, my students had forgotten (or claimed to have forgotten) much of what they “learned.”
When I reflected on this, I realized that I had expected the students to master these concepts, skills, and vocabulary without a meaningful context for them. It seemed difficult for them to apply processes introduced at the beginning of the year to an activity weeks later. I was certainly teaching the material (with the lesson plans to prove it), and the students seemed to know the material at the time. But they weren’t learning it well enough to apply it to new activities without a lot of review and re-teaching.
So in the following years I changed my approach. I decided to introduce only those SI/metric measures that are commonly used: kilograms, grams, and milligrams; liters and milliliters; kilometers, meters, centimeters, and millimeters. That’s it. I mentioned that other units such as decigrams or centiliters exist but are seldom used. (I’ve traveled a lot in Europe, Canada, and Australia and I never saw anything measured in hectograms or kiloliters!)
I introduced or reinforced these units within the context of investigations, rather than as separate and isolated topics of instruction. When we came to an investigation requiring liquid measurements, we first practiced with graduated cylinders and discussed the relationship between milliliters and liters. Students had a section in their notebooks for notes and drawings on measurements that they could use as reminders in future activities. I also found that students knew more than I had assumed.
We didn’t spend time on problems converting miles to kilometers or grams to ounces. It’s not worth it, and now most smart phones, tablets, and computers have apps that do these conversions. Students should know which SI units correspond to the ones they are more familiar with in the United States. For example, in much of the world meat and butter are sold in kilograms instead of pounds, distances between places in kilometers, gasoline and milk in liters, and so on.
I always had a few students say “Why do we have to measure this way?” which is a good question. I would mention that science research around the world uses SI measurements. The United States, Myanmar, and Liberia are the only nations in the world that do not use SI as the official system of weights and measures. But I clinched the discussion by asking students, “Who likes to play with fractions?” Very few hands were raised. When the students compared adding 1/8 inch and 3/16 inch vs. adding 3 mm and 4 mm they were convinced.
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