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# International System of Units

## Discusses two different systems of measurement and how to convert between them.

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SI Units: A Cautionary Tale

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 .

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
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 one-hundredth (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 one-tenth (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
Unit Conversions
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.

### Review

1. What does SI stand for?
2. Why is it important for scientists and engineers to adopt a common system of measurement units?
3. How many grams equal 1 kilogram?
4. What fraction of a meter is a millimeter?