Download Part I - TTU Physics

Chapter 1: Introduction
Physics
The most basic of all sciences!
• Physics:
The “Parent” of all sciences!
• Physics:
The study of the behavior & the
structure of matter & energy & of
the interaction between them.
The Purpose of Physics
• What does the word physics mean?
• A connection with natural philosophy.
• It is organized around a collection
of natural laws
• It tries to predict how “the world
works.”
• It tries to understand why “the
world works the way it does.”
What is Physics?
• Physics is the science of matter &
energy & interactions between them
• Matter & Energy are fundamental to all
areas of science.
• Physics is a foundational subject
• The principles of Physics form the basis
of understanding other sciences.
• Physics allows us to understand things
from very large to very small.
What is Physics?
• It is the study of the natural or
material world & phenomena
• The meaning of “physics” comes
from the Greek word for nature
• Historically, it was “Natural Philosophy”
• It is the oldest science
• Historically, all scientists were
Physicists.
Studying Physics
• The goal is to predict & understand
how the universe works
• It is organized around physical laws
– What do the laws say?
(Newton’s Laws of Motion!)
– How can we apply the laws to new
situations?
• Mathematics
– The laws of physics are generally expressed
mathematically
Sir Isaac Newton
• Mechanics is the main
physics area studied
in this course.
• The Laws of Mechanics
were developed by
Sir Isaac Newton
1642 - 1727
• Newton’s Laws of Motion
– Apply to a wide variety of
macroscopic objects
Sub Areas of Physics
• This course: Physics 1403
Physics of the 16th & 17th Centuries:
– Motion (MECHANICS) (most of our time!)
– Fluids & Waves
• The next course: Physics 1404
Physics of the
th
18
&
th
19
–Electricity & Magnetism
–Light & Optics
Centuries:
Sub Areas of Physics
• Advanced courses
Physics of the 20th & 21st Centuries!
–Relativity, Quantum Mechanics,
Atomic Structure, Condensed Matter,
Nuclear Physics, Particle Physics,
Astrophysics, Cosmology,….
These are the most interesting & the
most relevant to modern technology!
“Classical” Mechanics
Mechanics: “Classical” Mechanics
“Classical” Physics:
“Classical”   Before the 20th Century
– The foundation of pure & applied
macroscopic physics & engineering!
• Newton’s Laws of Motion
+ Boltzmann’s Statistical Mechanics
(+ Thermodynamics)
+ Maxwell’s Electromagnetism:
 Describe most of macroscopic world!
But,
• For objects at high speeds (v ~ c) we
need Special Relativity: (c = speed of light)
(Early 20th Century: 1905)
• For objects with small sizes (atomic &
smaller) we need Quantum Mechanics:
(1900 through ~ 1935)
• Our focus will be on
“Classical” Mechanics:
(17th & 18th Centuries)
Still useful today!
“Classical” Mechanics
• The physics in this course will be limited to
macroscopic objects moving at speeds v much, much
smaller than the speed of light c = 3  108 m/s. As
long as v << c, our discussion will be valid.
So, we will work
exclusively in
the gray region
in the figure.
“Mechanics”
• The science of HOW objects move
(behave) under given forces.
• (Usually) Does not deal with the
sources of forces.
• Answers the question:
“Given the forces, how
do objects move”?
Physics: General Discussion
• The Goal of Physics (& all of science)
is to quantitatively & 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 Research &
IMAGINATION!!
Physics & Its Relation to Other Fields
The “Parent” of all Sciences!
• The foundation for & connected to
ALL branches of science & engineering.
• Also useful in everyday life & in
MANY professions
– Chemistry
– Life Sciences (Medicine also!!)
– Architecture
– Engineering
– Various technological fields
Physics Principles are used in many
practical applications, including construction.
Communication between Architects &
Engineers is essential to avoid disaster!!!
The Nature of Science
• Physics is an EXPERIMENTAL science!
• Experiments & Observations:
– Important first steps toward scientific theory.
– It requires imagination to tell what is important.
• Theories:
– Created to explain experiments &
observations. Will also make predictions
• Experiments & Observations:
– Will tell if predictions are accurate.
• No theory can be absolutely verified
– But a theory CAN be proven false!!!
Theory
• A Quantitative (Mathematical)
Description of experimental observations.
• Not just WHAT is observed but WHY it’s
observed as it is & HOW it works the way it does.
Tests of Theories:
• Experimental observations:
More experiments, more observations.
• Predictions:
Made before observations & experiments.
Model, Theory, Law
• Model: Analogy of a physical phenomenon
to something we are familiar with.
• Theory: More detailed than a model.
•
Puts the model into mathematical language.
Law: A concise & general statement
about how nature behaves. Must be verified
by many, many experiments! Only a few laws.
Not comparable to laws of
government!
How does a new theory get accepted?
• It’s Predictions:
Agree better with data than those of an old theory
• It Explains:
A greater range of phenomena than the old theory
Example
• Aristotle: Believed that objects would return to rest
once put in motion.) (He was 100% wrong about everything
he said about physics!)
• Galileo: Realized that an object put in motion would
stay in motion until some force stopped it.
• Newton: Developed his Laws of Motion to put
Galileo’s observations into mathematical language.
Measurement & Uncertainty;
Significant Figures
No measurement is exact; there is always
some uncertainty due to limited instrument
accuracy & difficulty reading the results.
The photograph
illustrates this – it would
be difficult to measure
the width of this 24 to
better than a
millimeter.
Measurement & Uncertainty
• Physics is an EXPERIMENTAL science!
– It finds relations between physical quantities.
– It expresses those relations in the language of
mathematics. (LAWS & THEORIES)
• Experiments are NEVER 100% accurate.
There is always an uncertainty
in the final result.
• This is known as Experimental Error.
• It is common to state this precision (when it
is known).
• Consider a simple measurement of the
width of a board. Find 23.2 cm.
• However, measurement is only accurate
to 0.1 cm (estimated).
Write the width as (23.2  0.1) cm
 0.1 cm  Experimental uncertainty
Percent Uncertainty:
 (0.1/23.2)  100   0.4%
Significant Figures
Significant Figures (“sig figs”) 
The number the number of reliably
known digits in a number.
• Its usually possible to tell the number of
significant figures by how the number is written:
23.21 cm has 4 significant figures
0.062 cm has 2 significant figures
(initial zeroes don’t count)
80 km is ambiguous: It could have 1 or 2
significant figures. If it has 3, it should be
written 80.0 km
Sig Figs in Calculations With Numbers
• Multiplying or dividing numbers:
The number of sig figs in the result 
the same number of sig figs as the
number with the fewest sig figs that
is used in the calculation.
• Adding or subtracting numbers:
The answer is no more accurate than
the least accurate number used.
• Example
(Not to scale!)
• Area of a board:
dimensions 11.3 cm  6.8 cm
• Area = (11.3)  (6.8) = 76.84 cm2
11.3 has 3 sig figs , 6.8 has 2 sig figs
76.84 has too many sig figs!
Proper number of sig figs in the answer = 2
• So, round off 76.84 & keep only 2 sig figs
Reliable answer for area = 77
2
cm
Sig Figs
• General Rule: The final result of
multiplication or division should have
only as many sig figs as the number
with least sig figs in the calculation.
NOTE!!!!
All digits on your calculator are
NOT significant!!
• Calculators will not give you the
right number of sig figs. They
usually give too many, but
sometimes give too few
(especially if there are trailing
zeroes after a decimal point).
• The top calculator shows the
result of
2.0 / 3.0
• The bottom calculator shows the
result of
2.5  3.2.
Example: Significant Figures
• Using a protractor, you measure an angle of 30°.
(a) How many significant figures should you quote
in this measurement?
(b) Use a calculator to find the cosine of the angle you
measured.
(a) Precision ~ 1° (not 0.1°).
So 2 sig figs & angle is
30° (not 30.0°).
(b) Calculator: cos(30°) =
0.866025403. But angle
precision is 2 sig figs so
answer should also be 2
sig figs. So cos(30°) = 0.87
Powers of 10 (Scientific Notation)
READ Appendix B.3
• It is common to express very large or very
small numbers using powers of 10 notation.
Examples:
39,600 = 3.96  104 (moved the decimal 4
places to the left)
0.0021 = 2.1  10-3 (moved the decimal 3
places to the right)
PLEASE USE SCIENTIFIC
NOTATION!!
USE SCIENTIFIC NOTATION!!
• This is more than a request!! I’m making it
a requirement!!
When appropriate, I want to see powers
of 10 notation on exams!!
• For large numbers, like 39,600,
I want to see 3.96  104 & NOT 39,600!!
• For small numbers, like 0.0021,
I want to see 2.1  10-3 & NOT 0.0021!!
On exams, you will lose points if you
don’t do this!!
Accuracy vs. Precision
• Accuracy is how close a measurement
comes to the accepted (true) value.
• Precision is the repeatability of the
measurement using the same instrument
& getting the same result!
It is possible to be accurate without
being precise and to be precise
without being accurate!
Some General Comments on
Physics Problem Solving
• Problem Solving is the process of applying
a general physical law to a particular case
• Problem Solving is an essential part of physics
• Problem Solving takes a lot of practice!
The only way to learn physics is
to DO physics by solving HUGE
numbers of problems!
Types of Problems
• In this course, we’ll solve various types
of problems:
• Quantitative Problems: Given
some numerical information, use
calculations to find some physical
quantity.
• Concept Checks: Test your general
understanding of a law & its
applications
Reasoning & Relationship
Problems
• Identify what important information
might be “missing”.
• Successfully dealing with these
types of problems is essential to
gaining a thorough understanding of
physics
Problem Solving Strategies
1. Recognize the key physics
principles needed.
–You need a conceptual understanding
of the physics laws, how they are
applied, & how they are interrelated
2. Sketch the Problem!
–Show the given information
–Generally this includes a coordinate
system
Problem Solving Strategies
3. Identify the important relationships
– Use the given information & the unknown
quantities to determine what laws apply
– This may involve several substeps
4. Solve for the unknown quantities
– Be careful to do the math correctly!
5. Check your calculations!
– What do your answers mean?
– Do your answers agree with common
sense?
Problem Solving Strategies
Most importantly,
Think about your
answers! Are they
reasonable?
Use common sense!!