Name ______Date: ______Period: ______

Using Metric measurement in Forensic Science

Why would we want the metric system?

Let's face it: The metric system is the better system!

Why better?

The metric system is an improvement over the English system in three major points:

  1. One unit of measurement for each physical quantity. No matter from which other quantities of the SI system derived, yields always the same unit.
  2. Scalability by prefixes - combinations of prefix and unit make for a plethora of convenient combined units.
  3. Decimal system - fractions are expressed in decimal notation. Not only makes this the notation unique, exhaustive and intuitively comparable, but also allows unit conversion without doing math - simply by shifts of the decimal point.

Let's compare the metric system with the English system
  1. One unit of measurement for each physical quantity

To compare both systems here, we will list all English Imperial units in use, and all SI units describing the same range of the same physical quantities. These are: Length, area, volume, mass, force, pressure, energy, power and temperature. For a fair comparison, we omit those SI units which are commonly used in both systems, like second, ampere, volt, ohm etc.

List all of the units used in the English Imperial system fro each of the following physical quantities:

  1. Length:inch,______
  2. Area:square inch, ______
  3. Volume:gallon,______
  4. Mass/Weight:pound,______
  5. Force:______
  6. Pressure:______
  7. Energy:__________
  8. Power:______
  9. Temperature:______

List the unit used for the same physical quantities in the metric system:

  1. Length:______
  2. Area:______
  3. Volume:______
  4. Mass:______
  5. Force:______
  6. Pressure:______
  7. Energy:______
  8. Power:______
  9. Temperature:______
  1. Scalability by prefixes

Human beings can handle small whole numbers (0 to 999) better than huge numbers (e.g. 190080000), decimals (e.g. 0.672) or fractions (e.g. 77/256). That's why you want units that are close to the size of the thing you are measuring, so the resulting value is most likely to be a small whole number.

The English system addresses this problem by specifying all kinds of distinctly named, arbitrarily sized units. This causes problems:

  • you have to remember all the unit names
  • remember the chaotic plethora of conversion factors between them
  • worst of all - multiply and divide by them!

Now be amazed how few numbers you need to remember to measure the same range of the same quantities (length, volume, mass) in the metric system:

Prefix / Symbol / In Words
micro / µ / millionth
milli / m / thousandth
centi / c / hundredth
deci / d / tenth
BASE UNIT
deca / D / ten
hecto / H / hundred
kilo / K / thousand
mega / M / million

As there is only one unit of measurement per physical quantity, conversions between units are merely conversions between prefixes. Once you can convert one prefix to another, you can practically convert all SI units among each other.

So how to convert between these prefixes?

Just shift the decimal point - no math required!

Prefix / Symbol / In Words / Shift Point
nano / n / billionth / 9
micro / µ / millionth / 6
milli / m / thousandth / 3
centi / c / hundredth / 2
deci / d / tenth / 1
BASE UNIT
deca / D / ten / 1
hecto / H / hundred / 2
kilo / K / thousand / 3
mega / M / million / 6
giga / G / billion / 9

To convert 1852 meters to kilometers, just shift the decimal point ______places left = ______kilometers! A snap!

To convert 196 meters to micrometers, just shift the decimal point ______places right = ______micrometers! Again a snap!

  1. Decimal system

The decimal system is superior to the fractional system, because

  1. You can immediately see which of two numbers is greater.
  2. You can easily add and subtract decimal numbers.
  3. Each number has exactly one representation(Unlike 1/2=2/4=3/6...)
  4. Decimals make the value's precision evident.

You already use the decimal system for calculating with money - so you know it anyway! As it's the easier system, why not forget all the convoluted fractions and just use decimals!