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## Units Of Measurement

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Title: Units Of Measurement

1
Units Of Measurement
• Mrs. Mawhiney

2
Measurement of mass, length and volume
• In the United States, we use a fairly awkward
system of measurement for most things - the
English system Scientists use the metric and SI
systems of units for the measurement of physical
quantities
• This system using standard units based on very
precisely known properties of matter and light
• Prefixes are used in from of the units to
indicate powers of ten

3
SI Units
Measurement Unit Symbol
Mass Kilogram kg
Length Meter M
Time Second s
Temperature Kelvin K
Quantity Mole mol
Energy Joule J
Pressure Pascal Pa
4
. Base Units
• Mass - the quantity of matter that a sample
contains
• Note that weight is a measure of the attraction
of gravity for a sample and it varies depending
on the distance of the mass to a planet or moon
• Scientists often speak imprecisely of the
weight of an amount of substance. They really
mean mass.

5
Basic SI units/Derived units
• Used to generate new Units
• Volume - space a given quantity of matter
occupies
• Volume - expressed in terms of length - m3
• m3 - an inconveniently large volume, so we use
liter (L one cubic decimeter)
• We often use a mL (1 cubic centimeter) for more
manageable amounts of matter

6
Converting between units
• The standard method to convert between two
different units is the factor-label or
dimensional analysis method
• Dimensional analysis converts a measurement in
one unit to another by the use of a conversion
factor
• Conversion factors are developed from
relationships between units

7
Measurements and Units
• Measurement - determines the quantity,
dimensions or extent of something
• 1.Consist of two parts
• a. a numerical quantity (1.23)
• b. a specific unit (meters)
• Unit - a definite quantity adapted to as a
standard of measurement

8
Features of Measured Quantities
• When we measure a number, there are physical
constraints to the measurement
• Instruments and scientists are not perfect, so
the measurement is not perfect (i. e., it has
error)
• The error in the measurement is related to the
accuracy and the precision of the measurement

9
Accuracy and Precision
• Accuracy how close the measurement is to the
true value (of course we have to know what the
true value is)
• Precision is a measure of how closely
individual measurements agree with one another.

10
Example Accuracy and Precision
11
Equations for Precision and Accuracy
• 1. Precision
• 2. Accuracy
• Absolute Error
• AE (True value-Avg Value) X 100
• True Value

12
Counting Significant Figures
• Atlantic / Pacific Method
• a. Absent Decimal- Start on atlantic side
of number cross out all zeroes until 1st
nonzero digit is reached, remaining digits are
significant
• b. Present decimal- start on the pacific
side of the number cross out all zeros until
the 1st nonzero digit Is reached, remaining
digits are significant

13
How many significant figures are in each of the
following measurements?
24 mL
2 significant figures
3001 g
4 significant figures
0.0320 m3
3 significant figures
6.400 x 104 molecules
4 significant figures
560 kg
2 significant figures
14
Scientific notation and significant figures
• 1. When using scientific notation the base
must be written with the correct number of
significant digits
• 2. All zeroes are significant when using
scientific notation