Download Chapter 2 - AHISD First Class

Scientific Method
• Logical approach to solving problems
• Observing is the use of the senses to obtain
information.
• Data may be
• Qualitative (descriptive): Quality
• Quantitative (numerical): Quantity
System is a specific…during the experiment.
You are study the reactions in a test tube:
The test tube is the System
Hypotheses
• Scientist make generalization based on data
• This is a Hypothesis
• If-Then statements
• Testing Hypotheses
• Controls: conditions constant
• Variables: conditions that change
Strength of hypothesis
Repetition and Replication
• Repetition : experiment will give the same results
when it is performed under the same conditions.
• Replication is the idea that experiments should be
reproducible by other scientists.
Scientific method continued
A theory is a broad generalization that explains a body
of facts or phenomena.
• example: atomic theory, collision theory
A model : it is often an explanation of how phenomena
occur and how data or events are related
Example: atomic model of matter
Scientific Method
Chapter 2
Section 2 Units of Measurement
SI Measurement
• Scientists all over the world have agreed on a single
measurement system called Le Système International
d’Unités, abbreviated SI.
• SI has seven base units
• most other units are derived from these seven
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Chapter 2
Section 2 Units of Measurement
SI Base Units
© Houghton Mifflin Harcourt Publishing Company
Chapter 2
Section 2 Units of Measurement
SI Base Units
Mass
• Mass is a measure of the quantity of matter.
• The SI standard unit for mass is the kilogram.
• Weight is a measure of the gravitational pull on
matter.
• Mass does not depend on gravity.
© Houghton Mifflin Harcourt Publishing Company
Chapter 2
Section 2 Units of Measurement
SI Base Units
Length
• Length is a measure of distance.
• The SI standard for length is the meter.
• The kilometer, km, is used to express longer
distances
• The centimeter, cm, is used to express shorter
distances
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Chapter 2
Section 2 Units of Measurement
Derived SI Units
• Combinations of SI base units form derived units.
• pressure is measured in kg/m•s2, or pascals
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Chapter 2
Section 2 Units of Measurement
Derived SI Units, continued
Volume
• Volume is the amount of space occupied by an
object.
• The derived SI unit is cubic meters, m3
• The cubic centimeter, cm3, is often used
• The liter, L, is a non-SI unit
• 1 L = 1000 cm3
• 1 mL = 1 cm3
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Chapter 2
Section 2 Units of Measurement
Derived SI Units, continued
Density
• Density is the ratio of mass to volume, or mass
divided by volume.
mass
m
density =
or D =
volume
V
• The derived SI unit is kilograms per cubic meter,
kg/m3
• g/cm3 or g/mL are also used
• Density is a characteristic physical property of a
substance.
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Chapter 2
Section 2 Units of Measurement
Derived SI Units, continued
Density
• Density can be used as one property to help identify a
substance
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Chapter 2
Section 2 Units of Measurement
Derived SI Units, continued
Sample Problem A
A sample of aluminum metal has a mass of
8.4 g. The volume of the sample is 3.1 cm3. Calculate
the density of aluminum.
© Houghton Mifflin Harcourt Publishing Company
Chapter 2
Section 2 Units of Measurement
Derived SI Units, continued
Sample Problem A Solution
Given: mass (m) = 8.4 g
volume (V) = 3.1 cm3
Unknown: density (D)
Solution:
mass
8.4 g
3
density =
=
=
2.
7
g
/
cm
volume
3.1 cm3
© Houghton Mifflin Harcourt Publishing Company
Chapter 2
Section 2 Units of Measurement
Conversion Factors
• A conversion factor is a ratio derived from the
equality between two different units that can be used
to convert from one unit to the other.
• example: How quarters and dollars are related
4 quarters
=1
1 dollar
1 dollar
=1
4 quarters
0.25 dollar
=1
1 quarters
1 quarter
=1
0.25 dollar
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Accuracy & Precision
in Measurement
Accuracy & Precision
• Accuracy:
• Precision:
• How close you are to the
• How finely tuned your
actual value
• Depends on the person
measuring
• Calculated by the formula:
% Error = (YV – AV) x 100 ÷ AV
measurements are or
how close they can be
to each other
• Depends on the
measuring tool
• Determined by the
number of significant
digits
Where: YV is YOUR measured Value &
AV is the Accepted Value
Accuracy & Precision
• Accuracy & Precision may be demonstrated
by shooting at a target.
• Accuracy is represented by hitting the bulls
eye (the accepted value)
• Precision is represented by a tight grouping
of shots (they are finely tuned)
Accuracy & Precision
Precision without
Accuracy
Accuracy without
Precision
No Precision &
No Accuracy
Accuracy - Calculating % Error
How Close Are You to the Accepted
Value (Bull’s Eye)
Accuracy - Calculating % Error
• If a student measured the room width at
8.46 m and the accepted value was 9.45 m
what was their accuracy?
• Using the formula:
% error = (YV – AV) x 100 ÷ AV
• Where YV is the student’s measured value &
AV is the accepted value
Accuracy - Calculating % Error
• Since YV = 8.46 m, AV = 9.45 m
• % Error = (8.46 m – 9.45 m) x 100 ÷ 9.45 m
•
=
0.99 m
x 100 ÷ 9.45 m
•
=
99 m
÷ 9.45 m
•
=
10.5 %
• Note that the meter unit cancels during the division
& the unit is %. The (-) shows that YV was low
• The student was off by almost 11% & must
remeasure
• Acceptable % error is within 5%
•Acceptable error is +/- 5%
•Values from –5% up to 5% are acceptable
•Values less than –5% or greater than 5% must be remeasured
remeasure -5%
5% remeasure
Significant Digits
How to Check a Measurement for
Precision
Significant Figures
“Sig Figs” are the actual numbers used to
represent a measurement read from an
Instrument.
Numbers that are considered significant are all
of the numbers that can be directly read from
the numbers on an instrument plus one
estimated.
The estimated digit will always be the last
digit.
“IF DOT, THEN RIGHT; IF
NOT THEN LEFT”
Which digits are Significant?
All non-zero (1-9) digits are significant.
Some zeros are Significant, and some are not!
To determine if zeros are significant, use this
simple saying that allows you to draw an arrow to
“cross out” zeroes that are ___NOT________
significant.
The arrow stops when it reaches a nonzero digit.
“IF DOT, THEN RIGHT; IF
NOT THEN LEFT”
How many significant digits are in 0.090?
There is a decimal, so the arrow starts
outside of the number and points to the right.
The first nonzero digit the arrow reaches is a
9, making it, and any digit to the right of it
significant.
Therefore, there are ___two____ sig figs in
0.090.
“IF DOT, THEN RIGHT; IF
NOT THEN LEFT”
How many sig figs are in the number 20400?
There is no decimal, so the arrow starts
outside of the number and points Left.
The first nonzero the digit the arrow reaches
is the 4, making it and any digit to the left of
it significant
Therefore, there are 3 sig figs in the number
20,400
“IF DOT, THEN RIGHT; IF
NOT THEN LEFT”
• Identify the number of significant figures in
•
•
•
•
•
•
the flowing values:
3.909
3,450,000
8.880
0.0670
1,367
0.0002
More Practice with sig figs
How many sig figs?
4,500,000 2
71.29
4
.00900
3
4
2000.
1.3 x 102 2
509
3
121.00 5
1004 4
Calculations with Measurements
using Significant Figures
• When adding or subtracting the answer
should be rounded to the fewest decimal
places.
121.34 g + 1.562 g = 122.902 g → 122.90
• When multiplying or dividing, the answer
must have the least number of sig figs.
5.0 m x 6.32 m = 31.6 m → 32 m
What about the rounding?
When items have been rounded instead of
measured, or for an exactly defined
quantity, there are considered to be an
Infinite number of sig figs
Significant Digits & Precision
•What is the length of the bar?
• How many digits are
there in the
measurement?
• All of these digits are
significant
• There are 3 sig figs.
0
1
2
cm
Length of Bar = 3.23 cm
3
4