Main->Readings->4th Grade Readings->Matter- >Part 2: Measuring Matter
Vocabulary |
When people first started measuring things, they did not have rulers. To measure length, they measured things against their own bodies. An inch was about the width of an adult thumb. The foot was about the length of a human foot. There were problems with this. Different people are different sizes.
Another problem was that you had to remember how to convert different units for the same thing. For instance, when measuring volume, you had to remember that a gallon was divided into four quarts, and each quart into two pints, and each pint into sixteen ounces. When measuring weight, a pound was divided into twelve ounces by some people and sixteen ounces by others, and there were even different kinds of "pounds."
If you find this confusing, that is not surprising.
In 1790, the French decided to make things simpler. They started something called the metric system. The metric system has one main unit for each kind of measurement, and the only arithmetic you need is multiplying and dividing by ten, a hundred, or a thousand.
Multiplying a number by ten, or even by a thousand, is easy. If you want to multiply something by ten, you add a zero to the end. 6 times 10 is 60. It's just that simple.
And if you want to divide something by 10, you take away a zero. 50 divided by 10 is 5.
85 times a million? No problem. 1,000,000 has six zeroes. The answer is 85,000,000.
Another reason the metric system is simple is that it uses units based on things everybody can agree on, not the length of your arm or the size of your foot.
It is not surprising most of the world uses the metric system now.
The main unit of the metric system is the meter (m). The meter measures length. When they planned it, the French first set the meter to be one ten-millionth of the distance from the equator to the North Pole.
The meter was supposed to be 1/10,000,000
of the distance from the equator to the North Pole.
They were a little bit off, because they couldn't measure exactly enough, but you can see it is a better standard than somebody's thumb because there is only one Earth. These days they measure the meter by the speed of light, which is the same everywhere in the universe.
A meter is similar in length to a yard, and that is the last time we will mention yards, feet, inches, or any other "English system" measurements. The metric system is easy if you do not bother trying to convert.
The metric system uses prefixes. A prefix is a group of letters at the front of a word which changes the meaning of the word.
How can you measure something smaller than a meter? You can use centimeters. Centi- is a prefix meaning hundredths, so a centimeter is one-hundredth of a meter. Suppose you want to measure something much larger? How about the kilometer! Kilo- is a prefix meaning thousands, so a kilometer is a thousand meters.
There are many other prefixes besides kilo- and centi- in the metric system. For instance, the center of the atom is anywhere from 1 to several femtometers across. Femto- means 1/1,000,000,000,000,000! Scientists these days need the metric system because they have to measure things that are very tiny and things that are very big. You will only need to know a few of these for now. The most commonly used prefixes are in the table below:
| Metric Prefixes | |
|---|---|
| Prefix | How much |
| kilo- | 1,000 (thousand) |
| hecto- | 100 (hundred) |
| deka- | 10 (ten) |
| deci- | 1/10 (tenth) |
| centi- | 1/100 (hundredth) |
| milli- | 1/1000 (thousandth) |
Hecto- and deka- are not used as often in everyday life.
Remember the meter is the basic part of the metric system. So how do you measure volume with a meter? You still use centimeters. This is because you can measure the volume of a regular object by measuring the length of its sides. A cube 10 centimeters in length, width, and height is called a liter. A liter (L) is the metric unit used to measure volume.
Measuring mass is also easy to do in the metric system. A kilogram (kg) is about the weight of a liter of water, and a gram (g) is the weight of one cubic centimeter of water. That is, if you weighed a cube of water one centimeter on each side, it would weigh about a gram.
As you remember, you measure mass on a balance. So if you want to know how many grams something weighs, you put it on a balance and place objects whose mass you know on the other side.
Most of the world uses the metric system. In the United States, we often use the old system too, though we are gradually switching over to the metric system. This means that many things have to be specially made to sell in the United States, because we don't understand things like kilometers and kilograms and we still measure volume in ounces, cups, pints, and quarts.
However, all scientists use the metric system. This is because science is world-wide. Every scientist has to be able to talk to every other scientist, and the metric system is part of how they do it. Also, the metric system is more accurate, and science depends on getting things right. we use the metric system in science class.
Here is a list of common metric units and the abbreviations we have for them.
| Metric Units | ||
|---|---|---|
| Unit | Abbreviation | What it measures |
| meter | m | length |
| centimeter | cm | length |
| millimeter | mm | length |
| cubic centimeter | cc | volume |
| liter | L | volume |
| milliliter | mL | volume |
| gram | g | mass |
| kilogram | kg | mass |
Whenever you measure something, it is important to give two things: first, the number you found, and second, the units you used to measure with. You can say something is 5, but is it 5K, 5 g, or 5 mL? Units make a difference.
Questions: For your first assignment of the week, answer these questions in complete sentences on a sheet of loose-leaf paper, with a proper header:
Notes: For your second assignment of the week, in your journal on the next clean page, write the vocabulary words from this section and their definitions.
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Go on to Part 3: Describing Matter
This page last modified August 15, 2002
Copyright ©2000 Delia Marshall Turner. All rights reserved.
Questions? Send me a note at dturner@haverford.org