1.25: International System of Units
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, or SI. 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 the Table below.
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 
centi (c)  0.01  centimeter (cm) = 0.01 m 
milli (m)  0.001  millimeter (mm) = 0.001 m 
micro (µ)  0.000001  micrometer (µm) = 0.000001 m 
nano (n)  0.000000001  nanometer (nm) = 0.000000001 m 
Q: What is the name of the unit that is onehundredth (0.01) of a meter?
A: The name of this unit is the centimeter.
Q: What fraction of a meter is a decimeter?
A: A decimeter is onetenth (0.1) of a meter.
Unit Conversions
In the table below, two basic SI units are compared with their English system equivalents. You can use the information in the table to convert SI units to English units or vice versa. For example, from the table you know that 1 meter equals 39.37 inches. How many inches are there in 3 meters?


 3 m = 3(39.37 in) = 118.11 in

Measure  SI Unit  English Unit Equivalent 

Length  meter (m)  1 m = 39.37 in 
Mass  kilogram (kg)  1 kg = 2.20 lb 
Q: Rod needs to buy a meter of wire for a science experiment, but the wire is sold only by the yard. If he buys a yard of wire, will he have enough? (Hint: There are 36 inches in a yard.)
A: Rod needs 39.37 inches (a meter) of wire, but a yard is only 36 inches, so if he buys a yard of wire he won’t have enough.
Summary
 The measurement system used by most scientists and engineers is the International System of Units, or SI. There are seven basic SI units, including units for length and mass.
 If you know the English equivalents of SI units, you can convert SI units to English units or vice versa.
Vocabulary
 SI: International System of Units, which is used by most scientists.
Practice
Do the interactive unit conversion quiz at this URL. You can check your answers online.
http://www.quia.com/quiz/2611894.html
Review
 What does SI stand for?
 Why is it important for scientists and engineers to adopt a common system of measurement units?
 How many grams equal 1 kilogram?
 What fraction of a meter is a millimeter?
 How many pounds equal 5 kilograms?
Image Attributions
 Describe the International System of Units (SI).
 Convert units between International and English systems.