Untitled LessonSI

International System of Units (SI)

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1.2 International System of Units (SI)
Difficulty Level: At Grade | Created by: SD25 P
Last Modified: Aug 15, 2018

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 Tablebelow.

[Figure 1]

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International System of Units (SI)

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Prefixes of SI Units

Prefix

Multiply Basic Unit ×

Basic Unit of Length = Meter (m)

kilo- (k)

1000

kilometer (km) = 1000 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 Tablebelow, 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.

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International System of Units (SI)

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If you know the English equivalents of SI units, you can convert SI units to English units
or vice versa.

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?

5. How many pounds equal 5 kilograms?

Suggested Practice

PLEX SI Base Units

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