Basic Math and Pre-Algebra For Dummies (72 page)

Like the English system, the metric system provides units of measurement for distance, volume, and so on. Unlike the English system, however, the metric system builds these units using a
basic unit
and a set of
prefixes.

Table 
15-1
shows five important basic units in the metric system.

Table 15-1 Five Basic Metric Units

 For scientific purposes, the metric system has been updated to the more rigorously defined
System of International Units (SI).
Each basic SI unit correlates directly to a measurable scientific process that defines it. In SI, the kilogram (not the gram) is the basic unit of mass, the kelvin is the basic unit of temperature, and the liter is not considered a basic unit. For technical reasons, scientists tend to use the more rigidly defined SI, but most other people use the looser metric system. In everyday practice, you can think of the units in Table 
15-1
as basic units.

Table 
15-2
shows ten metric prefixes, with the three most commonly used in bold and italicized (see Chapter
14
for more information on powers of ten).

Large and small metric units are formed by linking a basic unit with a prefix. For example, linking the prefix
kilo-
to the basic unit
meter
gives you the
kilometer,
which means 1,000 meters. Similarly, linking the prefix
milli-
to the basic unit
liter
gives you the
milliliter,
which means 0.001 (one thousandth) of a meter.

Here's a list giving you the basics:

• Units of distance:
  The basic metric unit of distance is the meter (m). Other common units are millimeters (mm), centimeters (cm), and kilometers (km):
  

  

• Units of fluid volume:
  The basic metric unit of fluid volume (also called capacity) is the liter (L). Another common unit is the milliliter (mL):
  

  

  Note:
  One milliliter is equal to 1 cubic centimeter (cc).
  

• Units of mass:
  Technically, the metric system measures not weight, but mass.
  Weight
  is the measurement of how strongly gravity pulls an object toward Earth.
  Mass,
  however, is the measurement of the amount of matter an object has. If you traveled to the moon, your weight would change, so you would feel lighter. But your mass would remain the same, so all of you would still be there. Unless you're planning a trip into outer space or performing a scientific experiment, you probably don't need to
  know the difference between weight and mass. In this chapter, you can think of them as equivalent, and I use the word
  weight
  when referring to metric mass.
  

  The basic unit of weight in the metric system is the gram (g). Even more commonly used, however, is the kilogram (kg):
  

  

  Note:
  1 kilogram of water has a volume of 1 liter.
  

• Units of time:
  As in the English system, the basic metric unit of time is a second (s). For most purposes, people also use other English units, such as minutes and hours.
  

  For many scientific purposes, the second is the only unit used to measure time. Large numbers of seconds and small fractions of sections are represented with
  scientific notation,
  which I cover in Chapter
  14
  .
  

• Units of speed:
  For most purposes, the most common metric unit of speed (also called velocity) is kilometers per hour (km/hr). Another common unit is
  meters per second
  (m/s).
  
• Units of temperature (degrees Celsius or Centigrade):
  The basic metric unit of temperature is the
  Celsius degree
  (°C), also called the Centigrade degree. The Celsius scale is set up so that, at sea level, water freezes at 0°C and boils at 100°C.
  

   Scientists often use another unit — the kelvin (K) — to talk about temperature. The degrees are the same size as in Celsius, but 0 K is set at
  absolute zero,
  the temperature at which atoms don't move at all. Absolute zero is approximately equal to –273.15°C.
  

Most Americans use the English system of measurement all the time and have only a passing acquaintance with the metric system. But metric units are being used more commonly as the units for tools, footraces, soft drinks, and many other things. Also, if you travel abroad, you need to know how far 100 kilometers is or how long you can drive on 10 liters of gasoline.

In this section, I show you how to make ballpark estimates of metric units in terms of English units, which can help you feel more comfortable with metric units. I also show you how to convert between English and metric units, which is a common type of math problem.

 
When I talk about
estimating,
I mean very loose ways of measuring metric amounts using the English units you are familiar with. In contrast, when I talk about
converting,
I mean using an equation to change from one system of units to the other. Neither method is exact, but converting provides a much closer approximation (and takes longer) than estimating.

One reason people sometimes feel uncomfortable using the metric system is that, when you're not familiar with it, estimating amounts in practical terms is hard. For example, if I tell you that we're going out to a beach that's
mile away, you prepare yourself for a short walk. And if I tell you that it's 10 miles away, you head for the car. But what do you do with the information that the beach is 3 kilometers away?