SI Units
The SI System of Units
 Scientists need to report data that
can be understood by other
scientists. They need standard
units of measurement
 In 1795, French scientists adopted
a system of standard units called
the metric system
SI Units
 In 1960, an international
committee of scientists met to
update the metric system
 The revised system is called the
Système Internationale d’Unités,
which is abbreviated SI
SI Units
 There are 7 base units in SI
 A base unit is a defined unit that
is based on a constant in the
physical world
 Therefore, a base unit is
independent of other units
International System of Units (SI)
Unit Prefixes
 The size of each base unit can be
increased or decreased by adding
prefixes to the base units
 This task is made easier because
the metric system is a decimal
system – based on units of 10
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Time
 The SI base unit for time is the
second (s)
 The frequency of microwave
radiation given off by a
133cesium atom is the physical
standard used to establish the
length of a second
Length
 The SI base unit for length is the
meter (m)
 A meter is the distance that light
travels through a vacuum in
1/299,792,458 of a second
Mass
 Recall that mass is a measure of the
amount of matter present
 The SI base unit for mass is the
kilogram (kg)
 A kilogram is approx 2.2 pounds
 1 kg ≈ 2.2 lb
 The kilogram is defined by a
platinum-iridium metal cylinder
Derived Units
 Not all quantities can be measured
with base units
 For example, the SI unit for speed is
meters per second (m/s)
 Notice that meters per second
includes two SI base units - the meter
divided by the second
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Derived Units
 A unit that is derived by a
combination of base units is
called a derived unit
Volume
 Volume is the space occupied by
an object
 Two other common quantities that
are measured in derived units are:
 volume
 The derived unit for volume is the
cubic meter (1 m3), which is
represented by a cube whose sides
are all 1 meter (1 m1) in length
 density
Volume
 This definition works well for solid
objects with regular dimensions, but
not well for liquids or gases with
irregular shapes
 The metric unit for volume equal to
one cubic decimeter is a liter (L)
Volume – SI derived unit for volume is cubic meter (m3)
Density
1 L = 1000 mL = 1000
cm3
1 mL = 1 cm3
=1
dm3
 Density is a ratio that compares the
mass of an object to the volume that
it occupies
 The units for density will follow the
following pattern:
 Density = (mass unit)/(volume unit)
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Density
 Density is a unique physical
property that can be used to
identify an unknown sample
Problem
a sample of pure aluminum has a
mass of 13.5 g and a volume of
5.0 cm3, what is its density?
 If
 Every sample of a pure element,
regardless of the sample size, has
exactly the same density
Problem Solving
 When you analyze a problem, you first
need to identify what the question is
asking your to solve for
 Then you decide on a strategy to solve
the problem using the information that
was given
 After you solve a problem, it is important
to look at your answer and decide if it
makes sense
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