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Let me try to answer this important question with an example.
If you were to walk from your home to your school and count your steps, you might report that the distance from your home to your school is 2112 steps or paces. Ifyou are a male student, this would probably be approximately one mile, using assumptions that I will explain later. If you are a female, it would probably besomewhat less than a mile because your pace would probably be a little shorter than your male friend's pace.
A pace is a unit of measurement
In this case, the pace would be a unit of measurement. However, it would not be a standard unit because the length of a pace for one student is oftendifferent from the length of a pace for a different student. A male student, for example, typically has a longer pace length than a female student.
Standard units
Standard units are units whose value has been set by some official agency that has the authority to set standards. One such standard unit is the mile,which as you may know is equal to 5280 foots (more correctly pronounced feet).
A foot is also a standard unit, which you may also know is equal to 12 inches. An inch is another standard unit.
The discrepancy between the pronunciation of the unit foot and its plural feet is an example of the need for more consistency in the use of units. Many physics books use a system of units called SI units . SI is an abbreviation for a French name, which I am unable to pronounce, andwhich is probably also not compatible with your screen reader and your Braille display.
I won't attempt to explain much about SI units. You can probably find a good explanation in your textbook, and if not, you can Google SI units and findhundreds of web pages that explain the system in varying levels of detail.
Tables of SI units
Most of those references will probably also provide tables for the units, but those tables may be partially incompatible with your screen reader andBraille display due to the extensive use of superscripts. Therefore, I will attempt to provide accessible tables in Figure 2 and Figure 3 .
Don't ignore the details
Note, however, that you should not ignore the online SI-unit tables altogether. They are likely to contain important information that I won't reproduce here, such as thefact that the meter is a unit of length and its value is the length of the path travelled by light in a vacuum during a time interval of 1/299792458 of a second.
Base units and derived units
When reading about SI units, you will find that they are often divided into base units and derived units. I will put the base units in Figure 2 and some sample derivedunits in Figure 3 .
| Figure 2 . SI base units. |
|---|
Base Quantity Name Symbol
length meter mmass kilogram kg
time second selectric current ampere A
thermodynamic temperature kelvin Kamount of substance mole mol
luminous intensity candela cd |
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