# Aerospace Micro-Lesson #15

## Metric Units of Measurement

If we want to speak of how much there is of something, or how far it is to some place, we need units of measurement. Definitions of units have changed through the ages and there is a movement afoot to change the definition of the kilogram. The amount of mass in a kilogram would not change appreciably, but the way to figure it out would change.

If we are going to measure lengths, we need something like a ruler. A ruler gives us a standard length that we can use as our basis to compare other lengths to. For this to work, all the rulers in use need to measure the same length as one foot.

Hundreds of years ago, the “foot” was actually the length of the king’s foot. If the king grew, or if the people got a different king, the length of the “foot” would change. This can cause confusion. A way to illustrate this is to mark off a length on the floor—perhaps count a number of floor tiles—and have the students pace off the length by putting their feet heel-to-toe. The different-sized students will come up with different numbers of “feet” for the same length.

Going beyond the brief introduction for K-2 and the discussion of the need for standard units of measurement, compare the use of feet and inches to metric units like centimeters and meters. One good exercise is to have some students use yardsticks and others use meter sticks to measure a few items around the classroom, and then compare their results. Which units of measure resulted in larger numbers – inches or centimeters?

You can also lead students through a quick introduction/review of common metric prefixes – “micro,” “milli,” “centi,” “deci,” “kilo,” and “mega.” Can they think of other instances where they have seen these same prefixes used? Some examples are microbe, microscope, microwave, millennium, millipede, centipede, century, decimal, decade, and megaphone.

*A good read-aloud on this topic is Millions to Measure by David M. Schwartz.

The metric system has seven basic units:

• The meter is used to measure lengths
• The kilogram is used to measure masses
• The second is used to measure times
• The ampere is used to measure electrical currents
• The kelvin is used to measure temperatures
• The mole is used to measure the amounts of things
• The candela is used to measure the brightness of light

All other measurements can be expressed in terms of these units. This might sound strange, so let’s look at how some other quantities we are familiar with can be expressed in terms of base units. How about speed? Speed is how far (distance) something travels in a particular length of time. So the metric units for speed are meters per second, or m/s (length divided by time). How about acceleration? Acceleration is how much speed something gains in a particular length of time. So, the units should be the units for speed divided by time – meters per second per second, or meters per second squared. Let’s try one more: force. Force is equal to mass times acceleration, so its units should be kilograms times meters divided by seconds squared – kg-m/s2. These are the correct units, but you can imagine that it would get annoying always to write kg-m/s2. For that reason, more complex units in the metric system are usually given shorthand names; in this case, the unit is called the Newton, after the famous English physicist Sir Isaac Newton. The metric system has many of these more complex derived units and they are often named for famous scientists. Other examples include the Joule (energy), Pascal (pressure) and Tesla (magnetism).

For more information you can consult the National Institute of Standards and Technology. Their site also has a complete list of the metric prefixes.