Transcript Chapter 1, Introduction

Chapter 1, Introduction
Day 1
By the end of today…
• You will be able to tell other people what
physics is all about.
• You will be able to explain the scientific
method.
• You will be able to identify the SI units for
the most common measurements.
Physics:
The most mathematical
of all sciences!
• Physics = The study of the natural world.
Examines matter and energy and how
they interact.
Things You Will Learn About in
Physics
Normal
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wiki/Image:Flyingsuperconduct
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velocity
gravity
friction
friction
Sub Areas of Physics
• This Semester:
– Motion (MECHANICS) (most of our time!)
– Gravitation, Energy
• Second Semester:
– Electricity & magnetism
– Light, sound, optics, and more
Physics: General Discussion
• Goal of Physics (& all of science): To
quantitatively and qualitatively describe
the “world around us”.
• Physics IS NOT merely a collection of facts
and formulas!
• Physics IS a creative activity!
• Physics  Observation  Explanation.
• Requires: Calculation & IMAGINATION
How do Physicists come up with
theories ?
• First, they observe a phenomenon in nature.
• They often use the scientific method:
1.
2.
3.
4.
5.
Recognize a problem
Make a hypothesis- an educated guess
Predict the consequences of the hypothesis
Perform experiments that test these predictions
Conclusion: Formulate the simplest, general
rule that organizes and explains the hypothesis,
prediction, and experimental outcome.
Scientific Method in Action!
Car Crash Investigation: On the following
slide, see if you can match the number or
numbers on the right to the letter on the left
Forensic
Science uses
Physics all the
time to solve
their problems.
Match the number or numbers on the right to the
letter on the left
Scientific Method
a) Observe and collect
data
b) Form and objectively
test hypotheses by
experiments
c) Interpret Results and
Revise Hypothesis if
necessary.
d) State Conclusions in
a form others can
evaluate
1) Investigator might order blood
alcohol test, check car parts, or
try to reproduce marks on the
road.
2) Investigator must reexamine
evidence and possibly revise
hypothesis.
3) Examine scene and fill out report.
4) Investigator goes to court,
reexamines evidence, and defends
his theory.
5) Maybe the driver fell asleep, was
drunk, speeding, a tire exploded,
breaks did not work,
The Scientific Attitude
• Theories in science are not fixed. They
may be supported by data and test results,
but they are not facts.
• Example: The Model of the Atom
• The reviewing and changing of theories is a
strength of science, and the heart of the
scientific method.
“No number of experiments can prove me
right; a single experiment can prove me
wrong.”
~ Albert Einstein
Units: The SI System
• All measured physical quantities have units.
• Units are VITAL in physics and must be
included all the time !!
• In this course (and in most of the modern world,
except the USA!) we will use (almost)
exclusively the SI system of units.
SI = “Systéme International” (French)
SI or “Metric” System
5 Most Commonly used SI units
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Length unit: Meter (m)
Time unit: Second (s)
Mass unit: Kilogram (kg)
Temperature unit: Celsius (C)
Electric Current unit: Amperes (A)
There are others not used as often in a
Physics class
Looking Ahead…
•
The rest of this week we will focus on
reviewing measuring techniques for
acquiring data.
•
We will also review some basic math (YIKES!)
that will be important for you to master.
•
Linear Motion will then be the first step in
understanding “Mechanics” – the broad
Physics topic that we will focus on for the
first half of this course.
Graphing 101: A Complete Review
1. What is an independent variable? Where
could you find it on a graph (which axis)?
A variable that you change or manipulate. Usually
it is graphed on the x-axis.
2. What is a dependent variable? Where could
you find it on a graph (which axis)?
A Variable that is not manipulated, but is observed
(and often changes) as the independent variable
is changed. Usually graphed on the y – axis.
What's Next? LAB
• We will focus on recording accurate Data
and making logical Conclusions.
Spring/Rubber Band Activity
Focus:
1. Identifying Variables
2. Collecting GOOD Data
3. Accurately Reporting Data (tables and
graphs)
4. Analyzing the Results
A BAD GRAPH!
What is wrong with that graph ?
• In your notebook, explain why the graph on
the preceding page was not a good graph.
Steps to Improve the Graph
•Give the graph a title.
•Place labels on the x and y axis.
•Show units on the x and y axis.
•Do not play "connect the dots". Use a
“best fit line” - a straight line which goes
through the points or a curve that tends
to follow them.
A Better Graph!
Y-Axis
“Line of Best Fit”
Dependent
Variable
X-Axis
Independent
Variable
Relationships Between
Variables
• The simplest relationship between
two variables is a straight line or
“linear” relationship.
y=mx+b
“slope-intercept”
equation shows this relationship!
m= slope (change in y divided by change in x)
Quick Review of Slope
Change in Y
Change in X
The slope of a line is defined as the
rise over the run, m = Δy / Δx.
Δ means change
Other Relationships Between
Variables
Inverse
Quadratic (Square)
y = ax2 + bx + c
homepage.mac.com/cbakken/proportions/summary.html
Concluding Chapter 1
You must be able to:
• Identify SI base units and Prefixes
• Convert measurements into scientific
notation
• Distinguish accuracy from precision
Recall from earlier this week:
5 Most Commonly used SI units
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Length unit: Meters (m)
Time unit: Second (s)
Mass unit: Kilograms (kg)
Temperature unit: Celsius (C)
Electric Current unit: Amperes (A)
Prefixes
• Sometimes we need to measure things that
are either very big or very small.
• In addition to the SI units, prefixes and
scientific notation can be used to describe
size.
Larger & smaller units defined from SI
standards by powers of 10 & Greek prefixes
How to use these prefixes
Powers of 10
(Scientific Notation)
• It is common to express very large or very
small numbers using powers of 10 notation.
• Examples:
39,600 = 3.96  103 = 3.96 x 10 x 10 x 10
(moved decimal 3 places to left)
0.0021 = 2.1  10-3 = 2.1 / 10 / 10 / 10
(moved decimal 3 places to right)
Example Problem 1
A housefly is about 5 millimeters
wide. How many meters is this?
Show your work in your notebook to be checked.
Example Problem 2
A football field is 48.8 meters wide.
How many cm is this ?
Show your work in your notebook to be checked.
Example Problem 3
The mass of a mosquito is found to be
0.01 grams. Express this mass in
kilograms using scientific notation.
Show your work in your notebook to be checked.
Accuracy vs. Precision
• Accuracy – how close a
measurement is to the “true” or
“accepted” value.
• Precision – how repeatable or
“consistent” measurements are.
Accuracy vs. Precision
Another Example
• 5 people measure a track stars time to be
49 seconds. If the correct or “true” value
for the time was 60 seconds, we might
say the measurements were “precise” (in
other words consistent), but not accurate.
• If 5 people measured on or between 59
and 61 seconds, we would say their
measurements were pretty accurate.
Precision (continued)
• Precision – the precision of a
measuring device is limited by
the intervals on that device.
Precision
• Example: see figure 11 page 17 -- a
scale or measuring stick with only
centimeters and half centimeters
marked, could be “precise” down
to a millimeter.
• Really we would be estimating the
millimeter part of the reading.
Section III Objectives
Students will be able to:
• Use dimensional analysis to solve
equations
• Use order of magnitude
estimations to check whether
answers are reasonable.
Dimensional Analysis Example
• Density = mass / volume
• How could we use dimensional
analysis to show that the equation:
Volume = Density / Mass is
invalid?