1.25 International System of Units
1.25 International System of Units
- Describe the International System of Units (SI).
- Convert units between International and English systems.
In 1999, NASA’s Mars Climate Orbiter, pictured here, burned up as it passed through Mars’ atmosphere. The
satellite was programmed to orbit Mars at high altitude and gather climate data. Instead, the Orbiter flew too low
and entered the red planet’s atmosphere. Why did the Orbiter fly off course? The answer is human error. The flight
system software on the Orbiter was written using scientific units of measurement, but the ground crew was entering
data using common English units.
SI Units
The example of the Mars Climate Orbiter shows the importance of using a standard system of measurement in
science and technology. The measurement system used by most scientists and engineers is the International System
of Units, orSI. There are a total of seven basic SI units, including units for length (meter) and mass (kilogram). SI
units are easy to use because they are based on the number 10. Basic units are multiplied or divided by powers of
ten to arrive at bigger or smaller units. Prefixes are added to the names of the units to indicate the powers of ten, as
shown in theTable1.4.
TABLE1.4: Prefixes of SI Units
Prefix Multiply Basic Unit× Basic Unit of Length = Meter (m)
kilo- (k) 1000 kilometer (km) = 1000 m
deci- (d) 0.1 decimeter (dm) = 0.1 m