2. Exam 1 due Monday, Sep. 18
11:59 PM
The exam will open at 12:01 am the previous day. You have a 48-
hour window to begin the exam.
The exam will be given via Blackboard. Special apps like
Respondus are NOT required.
Class will meet on exam day.
You are responsible for making sure you have access to a
computer with good wifi. I recommend using a computer on
campus. Computer labs include:
• Mullins Library (MULN)
• Arkansas Union (ARKU)
• Physics Library (PHYS 221)
3. Exam 1 format
Similar to clicker questions & homework
About 40 questions
Exam will be timed. You will have about 1 minute per
question.
4. How to Make a Cheat
Sheet
The best way to prepare for the exam is to make a
handwritten cheat sheet.
Write your cheat sheet as you study lecture slides. Use
your book to clarify difficult topics.
Use color – or a highlighter – to make the topic headings.
That will allow you to find information quickly during the
exam.
5. Exam 1 Content
What is covered?
Lectures 1 – 5b (Chapters 1-5)
What to study, in order of importance:
Lecture slides
Book chapters
6. Chapter 1
The Way of Science:
Experience and Reason
The Scientific Process
Geocentric vs. Heliocentric model
Retrograde motion
Stellar Parallax
Galileo & the phases of Venus
7. Evidence for the
heliocentric model
Stellar parallax
Phases of Venus
Moons of Jupiter
Kepler’s Laws predict the motion of the planets
across the background stars better than the
geocentric model
8. Chapter 2
Atoms: The Nature of Things
Chapter 2: Atoms – The Nature of Things
Democritus’s view of atoms
Dalton’s water experiments
Brownian Motion
Phases of matter (See Fig. 2.9)
Metric units of mass & length
9. Chapter 3
How Things Move: Galileo
Asks the Right Questions
What happens to an object in freefall
How things fall with air resistance vs.
without
Definition of acceleration
Law of Inertia
10. Chapter 4
Why Things Move As They Do
Law of Motion
Mass vs. Weight
Definition of Force
Definition of Net Force
Law of Force Pairs
11. Chapter 5
Newton’s Universe
To be in orbit is to be in freefall
Newton’s Theory of Gravity
Why does the Sun shine?
Why does a high mass star go supernova?
12. Go study!
This review covers many, but not all, of the topics on
Exam 1.
To study, condense all your class notes into a cheat
sheet.
Use your book to clarify vague or confusing
concepts.
Break a leg!
14. Sound off: Physics
Rocks!
Newton discovered gravity, y’all!
To be in orbit is to be in freefall.
More distant stars attract each other less.
More massive stars attract the best.
Physics rocks!
16. 5.3 Gravitational Collapse:
Evolution of the Solar System
We live in a Galaxy called
the Milky Way.
There are about 400 billion
stars in the Milky Way.
In addition, the Galaxy has
lots of gas clouds (nebulae).
https://www.nasa.gov/jpl/charting-the-milky-way-from-the-inside-out
17. Nebulae: clouds in space
A gas consists of freely
floating particles.
The gas in a nebula
consists mostly of:
• Hydrogen atoms (H)
• Hydrogen molecules (H2)
• Helium (He)
With traces of other
things, like
• Carbon monoxide (CO)
• Water vapor (H2O)
Horsehead Nebula
From Astronomy Picture of the Day
https://apod.nasa.gov/apod/ap171227.html
18. Gravity in a nebula
According to Newton’s Law of Gravity, every particle
in a nebula attracts every other.
The more distance between two particles, the weaker
their attraction.
So, if the particles are (on average) very far apart,
gravity won’t affect the nebula much.
However, if the particles are close together – that is, if
the gas is dense enough – the particles will pull
together.
In other words, the nebula will collapse under its own
weight.
19. Nebular Theory of Solar
System Formation
When a nebula collapses, it
forms a star and the planets,
asteroids, & comets that
surround it.
Most of the gas ends up at the
center of collapse (the star).
For example, where is the
mass in our Solar System
located?
• 99.8% The Sun
• 0.1% Jupiter
• 0.1% Everything else
HL Tauri, a protoplanetary disk
(that is, a young star and its planets in the
process of forming).
https://apod.nasa.gov/apod/ap141110.html
20. Stars a born in clusters
A nebula is usually much
larger than our Solar
System.
If a nebula is dense enough
to collapse into a star, it
usually forms thousands of
stars at the same time.
In other words, a single
nebula gives birth to a star
cluster. The Orion Nebula, a stellar nursery
https://apod.nasa.gov/apod/ap151229.html
21. Stars form in spiral
arms
The Milky Way is a spiral
galaxy.
The spiral arms are density
waves.
In other words, the gas is
more dense in the spiral
arms than in surrounding
space.
Therefore, most stars in our
Galaxy are formed in the
spiral arms.
Spiral Galaxy NGC 1232
https://apod.nasa.gov/apod/ap171226.html
22. Spiral arms are (kinda) like
sound
Sound waves are density
waves, like galactic spiral
arms.
Spiral arms are much larger
than sound waves, and
massive enough to have their
own gravity.
Astronomers sometimes
refer to spiral arms as heavy
sound.
Sound waves
http://www.mediacollege.com/audio/01/sound-waves.html
Spiral arms
https://apod.nasa.gov/apod/ap150502.html
23. How a nebula becomes a star
As the nebula collapses, it
becomes:
• Hotter, due to the Law of
Conservation of Energy (more
on this law later)
• Denser, because the particles
are being drawn toward one
another.
When the center of the nebula
becomes hot and dense enough,
nuclear fusion begins.
At that moment, a star is born.
Elias 2-27, a young star with
a protoplanetary disk
http://www.astronomy.com/news/2016/10/spiral-arms
24. Nuclear fusion
Nuclear fusion is the
joining together of
elements to form larger
elements.
In the Sun and most other
stars, this means turning
hydrogen (H) into helium
(He).
In most cases, nuclear
fusion releases energy.
Lots and lots of energy.
25. Why the Sun shines
Nuclear fusion
is why the Sun
shines today.
The Sun is NOT
on fire!
• Fire is a chemical reaction, which doesn’t have enough
energy to maintain the Sun’s glow.
• The Sun runs on nuclear power, not chemical reactions
like fire.
26. Fusion in the Sun
The Sun releases energy by fusing four hydrogen nuclei
(protons) into one helium nucleus.
0.7% of the protons’ mass is converted to energy
(sunshine) via Einstein’s famous equation E=mc2.
This process is called the proton-proton chain (the pp
chain).
27. Hydrogen Bomb
Nuclear fusion also powers the
most powerful weapon on Earth:
the hydrogen bomb.
To set off a hydrogen bomb is to
recreate the conditions inside the
core of the Sun, if only for an
instant.
In other words, a hydrogen bomb
is like a miniature star. Operation Castle
thermonuclear test
https://en.wikipedia.org/wiki/Thermonuclear_weapon
28. 5.4 How stars die
M1, the Crab Nebula, is the remnant of an exploding star.
https://apod.nasa.gov/apod/ap180104.html
29. Low-mass stars
Our Sun is a low-mass star.
It’s more massive than most
stars, but much less massive
than a high-mass star.
As of now, the Sun fuses
hydrogen into helium to
release energy.
30. As the Sun gets old…
In a few billion years the Sun will fuse helium
into carbon and oxygen.
31. Fusion in the Sun
As of now, Hydrogen fuses into helium.
Much later, helium will fuse into carbon, then oxygen.
32. As the Sun dies…
At the very end of its life, the Sun will expel its
outer layers out into space.
Its core will remain behind as an extremely
dense ball of carbon & oxygen called a white
dwarf.
A white dwarf has the mass of a star but is
compacted down to the size of Earth.
33. Planetary nebula: death
shroud of a Sunlike star
At the very end of its life,
the Sun will poof its outer
layers out into space,
forming a planetary
nebula.
A planetary nebula has
nothing to do with
planets (sorry).
The Sun’s core core will
remain behind as an
extremely dense ball of
carbon called a white
dwarf.
Helix Nebula with a white dwarf
at the center
https://apod.nasa.gov/apod/ap160920.html
34. White dwarf: the zombie
corpse of a Sunlike star
A white dwarf has the
mass of a star, but is
compacted down to the
size of Earth.
A fragment the size of
your thumb would
weigh as much as a car!
Why is it so dense? It
was compressed by the
weight of the Sun. In
other words, by gravity. Spirograph Nebula with a white dwarf
at the center
https://apod.nasa.gov/apod/ap170611.html
35. High mass stars
Stars with more than 10 times
the mass of the Sun meet a
more dramatic end.
Like our Sun, they spend most
of their years turning hydrogen
unto helium.
Near the end, they turn helium
into carbon & oxygen.
Our Sun stops there, but a high
mass star will continue…
The constellation Orion. The
left shoulder, the red
supergiant star Betelgeuse,
is a high mass star near the
end of its life.
https://en.wikipedia.org/wiki/Orion_(constellation)
36. High mass stars
Because a high-mass star is so massive, its outer
layers are heavy enough to compress its core even
more than the Sun.
Its core is therefore even hotter and more dense than
our Sun’s.
It will continue building heavier and heavier
elements…
37. Fusion in high mass stars
High core temperatures allow helium to fuse with
heavier elements.
38. Fusion in high mass stars
Helium capture builds carbon into oxygen, then
neon, then magnesium …
39. Fusion in high mass stars
Advanced reactions in stars make elements such as
silicon, sulfur, calcium, and … iron.
40. Iron hates to fuse.
For elements lighter than iron, nuclear fusion
releases energy.
In the core of the star, that energy holds up the
weight of the star.
Iron, on the other hand, does not release energy
when it fuses. It absorbs it.
Once Iron appears in the core, it sucks up the energy.
There is nothing left to hold up the weight of the
core, so it collapses under its own weight.
41. Iron kills the star.
Core temperatures in high mass stars allow fusion of
elements as heavy as iron. Iron triggers a supernova.
Within a few seconds, the star explodes.
42. Core-collapse
supernova
When iron appears in the core, the core goes dark.
Without light, there is nothing in the core to hold up
the weight of the star.
The outer layers fall inward at 1/4 the speed of light,
resulting in…
A shockwave (explosion) called a supernova.
43. Supernova: the death of
a high mass star
While a high mass star is exploding, it is as bright
as a galaxy.
In other words, it can outshine 100 billion other
stars combined.
The star’s outer layers return to space as a nebula.
44. A supernova is as bright
as a galaxy
Supernova
1987a, observed
in the Large
Magellanic
Cloud, a satellite
galaxy orbiting
our Milky Way.
45. A supernova leaves
behind a nebula
Crab Nebula,
remnant of the
supernova observed
by Chinese
astronomers in
1054.
46. Neutron Stars
When a high mass star goes supernova, its core gets
seriously compressed.
In most cases, the core ends up as a city-sized atomic
nucleus called a neutron star.
A neutron star is even more dense than a white dwarf.
A fragment the size of your thumb would weight 10,000
tons!
Neutrons emit pulses of radiation when they pulse. If the
pulses sweep across Earth, we refer to the neutron star as
a pulsar.
47. Black holes
Neutrons stars are compressed enough to have
extreme amounts of gravity. They would pull you
into spaghetti.
Most high mass stars become neutron stars upon
exploding, but the highest mass stars become black
holes.
A black hole is an object that is so dense, its
gravitational pull is too strong for anything to
escape, including light.
48. Recap: zombie corpses
of stars
Low-mass stars like our Sun become white dwarfs.
High-mass stars become neutron stars.
Extremely high-mass stars become black holes.
49. 5.5 The Newtonian
Worldview
According to Aristotle, physics was fundamentally
different in the heavens than on Earth.
Before Newton, the Catholic Church adopted the
worldview of Aristotle. “The Heavens,” which we
now call outer space, was considered (literally)
Heaven, the home of God and the Angels.
In Europe of the Middle Ages, God and the Angels
lived in the sky, directly above humans, and the laws
of physics were thought to be different there.
50. Newton felt that science
and religion are compatible.
Newton was devout, but he disagreed with the Church.
According to Newton, we live in a universe, where the
laws of physics apply equally in space and on Earth:
• The Law of Inertia
• Law of Motion
• Law of Force Pairs, and
• Law of Gravity
Heaven, then, was not a physical place directly above your
head. Rather, Heaven became a spiritual realm. You have
to die to get there.
51. Dualism
Newton’s view is now called dualism.
According to dualism, the Universe is governed by
physical laws, while the spiritual realm (heaven) is
governed by gods or God.
52. 5.6 Beyond Newton:
Limitation of Newtonian
Physics
Remember: science isn’t perfect. It just gets better
with time.
Newton’s Laws made better predictions than
previous scientific theories, so they were considered
superior…
But they aren’t perfect. We now have updated
theories.
53. When Newton’s Laws Fail
Newton’s physics has been shown to fail under extreme
circumstances, such as:
For objects that are smaller than an atom. For that, we
need quantum mechanics.
When objects move near the speed of light. For that, we
need Einstein’s Theory of Special Relativity.
For circumstances where we need Einstein’s Theory of
General Relativity. These circumstances include:
• Extreme gravity, like near a black hole
• Large distances, as in the Universe as a whole
• Extremely precise time measurements, as in Global
Positioning System
55. When modern physics fails
Neither General Relativity (GR), the physics of
extreme gravity, nor Quantum Mechanics (QM), the
physics of small particles, has ever been contradicted
by experiment.
However, they make contradictory predictions inside
a black hole.
• GR says the black hole’s mass should collapse into a
singularity, a point or ring of infinite density.
• QM says singularities cannot exist.
56. WTF is happening inside
a black hole?
Inside a black hole, either quantum mechanics or general
relativity (or both!) is wrong.
Light can’t escape from a black hole, so we can’t look
inside.
Since we can’t look inside, we can’t experiment. We can’t
tell if GR is correct, or QM is correct, or neither.
There are several hypotheses about what goes on inside a
black hole. We call this field of study quantum gravity.
57. Sound off: Physics
Rocks!
Aristotle said things fall faster if they’re heavy.
Newton said they fall by the Law of Gravity.
Einstein said it’s all General Relativity.
Einstein wasn’t perfect, just sublime.
Science isn’t perfect. It just gets better with time.
Physics rocks!
63. Look at more JWST
Images!
Link to the JWST Gallery
64. Review: Forces
Forces come in equal and
opposite pairs.
Objects always push one
one another with equal
force in opposite
directions. (Law of force
pairs)
The less massive object
accelerates more (Law of
motion)
Force on basketball = Force on tennis ball
but in the opposite direction
Tennis ball accelerates (bounces) more
66. Sound off: Physics
Rocks!
The Law of Force Pairs:
When I push a bear
The bear pushes back
With equal force, in fact.
67. Sound off: Physics
Rocks!
But I’m the one
Who falls on the grass
Because the bear
has most of the mass.
Physics rocks!
68. 5.1 The Idea of Gravity: The
Apple and the Moon
By analyzing the planets’ positions in the sky and the
motion of falling bodies, Isaac Newton (1642-1727)
discovered gravity:
The force that drops and apple to the ground also
keeps the Moon in orbit around the Earth.
Remember: Aristotle told us the laws of motion
(physics) in the heavens are different than on Earth.
But Newton said no! Physics is everywhere the same.
We live in a universe.
69. To orbit is to fall.
The Moon is falling toward
the Earth.
If it weren’t, it would keep
moving in a straight line.
Earth’s gravity accelerates
it – that is, changes its
direction.
Instead of moving in a
straight line, the Moon
orbits in an ellipse (a
slightly elongated circle).
Fig. 5.2
70. To orbit is to fall.
What’s the
difference
between orbiting
and falling?
We call it falling
when the ground
is in the way.
An object in orbit is
also in freefall.
Stephen Hawking in freefall.
https://en.wikipedia.org/wiki/Reduced-gravity_aircraft#/media/File:Physicist_Stephen_Hawking_in_Zero_Gravity_NASA.jpg
71. The faster you throw a rock,
the farther away it lands.
Therefore…
if you throw the
rock fast enough
(and if there are
no buildings or
mountains in the
way),
it will not land. It
will stay in orbit.
Fig. 5.3
Fig. 5.4
72. 5.2 Newton’s Theory of Gravity:
Moving the Farthest Star
Newton’s Theory of Gravity: Every object
in the Universe attracts every other.
The more massive the two objects, the stronger the
attraction.
The more distance between two objects, the weaker the
attraction.
73. Example: Earth and
Moon
The Earth and the Moon attract one another
gravitationally.
By the law of force pairs, each attracts the other with
equal force.
However, Earth is much more massive than the
Moon.
Therefore, the Moon accelerates more. The is, the
Moon orbits, while the Earth only wobbles a bit.
74. Example: Small objects
The microphone I’m holding is gravitationally
attracting the chair I’m sitting on.
The attraction is so small it can’t overcome friction,
so there is no acceleration.
Even if there were no friction, the acceleration would
be tiny.
However, such attraction between small objects has
been measured.
75. Equation of Newton’s
Gravity
The gravitational force between two objects depends on their masses
and the distance between them.
Note: This is much like the weight equation, F=mg, except it works in
space as well as on a planet.
F = Gm1m2
d2
76. F = Gm1m2
d2
Anatomy of the gravity
equation.
G is called the universal gravitational constant.
It is a very small quantity. The reason: it takes something
with the mass of the Earth to give you your weight.
Gravity is a very, very weak force.
77. Anatomy of the gravity
equation.
m1 and m2 are the masses of the objects attracting one
another.
m1 and m2 are in the numerator (top of the fraction).
That means the gravitational force rises with the masses.
The more massive the objects, the stronger their attractions.
F = Gm1m2
d2
78. Anatomy of the gravity
equation.
d is the distance between the two objects.
d is in the denominator (bottom of the fraction).
That means the gravitational force rises with the distance
decreases. The farther apart the objects, the less they
attract.
F = Gm1m2
d2
79. Gravity decreases with
distance squared.
The distance between two objects affects the
gravitational force more than either of their masses.
80. Caveats
Newton’s theory of gravity explained & predicted things
Aristotle’s physics could not.
Newton’s theory of gravity therefore replaced
Aristotle’s physics. However…
81. Caveat
Newton’s theory isn’t perfect. It’s good enough to
predict the positions of the planets, and to send probes
around the Solar System. But it fails under certain
circumstances. For example:
• When gravity is really strong (for example, near a
black hole)
• When you need to make extremely extremely precise
measurements (for example, Global Positioning
System)
82. Caveat
When you need to analyze motion under extremely strong
gravity, or to make extremely precise measurements, you
must turn to…
Einstein’s Theory of General Relativity! (Chapter 11)
Einstein updated Newton’s gravity. Relativity makes
correct predictions whenever Newton’s theory does…and
also under circumstances when Newton’s theory fails.
Remember: science isn’t perfect. It just gets better with
time.
83. Sound off: Physics
Rocks!
Newton discovered gravity, y’all!
To orbit is to be in freefall.
More distant objects attract less.
More massive objects attract best.
Physics rocks!
88. 4.4 Weight
Gravity is a force.
Definition: Weight is the
gravitational force
exerted on an object.
CAUTION: The word
weight is badly defined.
Different introductory
physics books have
different definitions.
https://en.wikipedia.org/wiki/Weight
89. Property of weight
Objects of equal mass
have equal weight, so
long as they feel the
same gravity.
In other words, as long
as they’re both on
Earth.
But … two objects of
the same mass have
different weights if one
is on Earth and the
other is on Mars.
Mars.nasa.gov
91. Equation of Weight
We use F for weight to remind us:
weight is a force.
Some textbooks use W for weight.
F = mg
92. Anatomy of an Equation
Remember: the equal sign is a balance.
What you do to one side, you have to do
to the other.
Since there’s no fraction, things on the
opposite side of the equal sign rise
together.
F = mg
93. Anatomy of an Equation
m is an object’s mass, in kilograms.
F and m rise together.
The more massive the object, the greater
the weight (assuming both objects are on
the same planet)
F = mg
94. Anatomy of an Equation
g stands for the acceleration due to gravity. It varies
from planet to planet, and from moon to moon.
On Earth, g = 9.8 m/s2.
On our Moon, g = 1.6 m/s2.
On Mars, g = 3.7 m/s2.
F = mg
95. Anatomy of an Equation
F and g rise together.
The more gravity and object feels, the
the heavier it will be.
F = mg
96. Caveat
This equation only applies to a small object falling near
the surface of a planet or a moon, where g is well defined.
The word weight is (usually) only applied to such cases. In
some books, weight is only used when something is
resting on a surface, as opposed to falling.
In space (say, when two stars are orbiting one another)
the equation gets a little more complicated.
F = mg
97. But wait…
If weight is a force, and forces make things
accelerate, then why am I not accelerating
downward?
Answer: Only a net force (an imbalance of forces)
makes things accelerate.
Since you’re not accelerating, there must be another
force balancing your weight.
98. The Normal Force
The normal force
balances your weight.
The normal force is how
the floor holds you up.
Since your weight
(gravity) pulls you
downward, the normal
force must point upward
in order to balance.
99. The Normal Force
If the floor is
unable to
provide enough
normal force to
balance gravity,
then you will
fall through.
Normal force FAIL
https://www.youtube.com/watch?v=7PA-GzpcgIA
100. 4.5 The Law of Force
Pairs
Thought experiment: Slap a table.
Smack!
Note that you apply a force to the
table.
Observation: your hand stings. Why?
When your hand applies a force to the
table, the table applies an equal force
back.
In other words, when you smack the
table, the table smacks you back!
http://www.criticalmiss.com/issue8/funwithdice1.html
101. 4.5 The Law of Force
Pairs
Thought experiment: Push on a wall.
Note that you apply a force to the
wall.
Observation: You push yourself
backward. Why?
When your hand applies a force to the
wall, the wall applies an equal force
back.
In other words, when you push the
wall, the wall pushes you back!
https://acac.com/charlottesville/news/snow-day-bodyweight-exercises/
102. 4.5 The Law of Force
Pairs
Thought experiment:
Discharge a firearm.
Your weapon applies a
force to the bullet.
Observation: Recoil.
Your weapon gets pushed
backward. Why?
When your weapon applies a force to the bullet, the bullet
applies an equal force back.
In other words, when the gun shoots the bullet, the bullet
pushes back!
http://www.futureproofstudios.com/
103. The Law of Force Pairs
Forces always come in equal
and opposite pairs.
In other words, if I push you, then you push me back with
equal and opposite force.
This is sometimes called Newton’s 3rd Law.
104. The Law of Force Pairs
Forces always come in equal
and opposite pairs.
Note: this does NOT mean you and I accelerate (bounce)
equally when we collide.
Remember the Law of Motion: given the same force, the less
massive object accelerates more.
When two objects collide or interact, they give each other the
same force. The less massive object accelerates more.
105. The Law of Force Pairs
Back to our thought experiment: Push
on a wall.
The wall applies an equal and opposite
force to you.
So, why do you bounce and the wall
doesn’t? (In other words, why do you
do practically all the accelerating?)
The more massive the object, the less
the acceleration.
The wall is attached to the ground. The
wall & ground are much more massive
than you!
https://acac.com/charlottesville/news/snow-day-bodyweight-exercises/
106. The Law of Force Pairs
Back to our thought
experiment: Discharge a firearm.
When you apply a force to the
bullet, the bullet applies an equal
force back.
Why does the bullet fly off at 3,000
feet per second, while you only recoil a little?
The more massive the object, the less the acceleration.
You and your weapon are much more massive than the bullet, so
the bullet accelerates much more.
http://www.futureproofstudios.com/
107. The Law of Force Pairs
New thought experiment: An apple
falling toward Earth.
Earth pulls on the apple gravitationally.
By the Law of Force Pairs, the apple
must pull the Earth with equal force!
So why does the apple do practically all
the falling (accelerating)?
The less massive object accelerates more.
Because the Earth has practically all the
mass!
108. Definitions
System: An object or group of one or
more objects.
Center of mass (a.k.a. center of gravity):
The balance point of an object or a
system.
109. 4.6 Newton Meets the
Automobile
https://www.amazon.com/Isaac-Newton-School-Driving-Physics/dp/0801874173
110. A car is a system.
Only an external force can
accelerate a system.
A car has moving parts that
exert forces on one
another, but …
The system as a whole
can’t exert a force on itself.
So, what accelerates a car?
111. What accelerates a car?
To accelerate forward, the car exerts a backward force on the
ground.
By the law of force pairs, the ground exerts a forward force
on the car.
In other words, the car pushes backward on the ground, and
the ground pushes forward on the car.
By the law of motion, the car does practically all the
accelerating because the ground has practically all the mass.
115. Vertical forces
Figure
4.16
Since the car doesn’t accelerate upward or
downward, the normal force must be
balancing the gravitational force.
Upward
normal
force,
applied by
the ground
Downward
gravitational
force
(weight),
applied by the
planet
120. Horizontal forces
Figure
4.16
Forward drive
force, applied
by the ground
Backward air
resistance, a
force applied by
the air
Backward
rolling
resistance, a
force applied by
the ground
Hint: rolling resistance is much greater when
tires are low. Air up your tires! You’ll save
between 5% and 15% at the gas pump.
122. Horizontal forces
Figure
4.16
If the car is moving at constant speed, it is not
accelerating. The forces are balanced.
If the car is
speeding up, the
drive force is
stronger than the
air resistance and
rolling resistance
combined.
123. Horizontal forces
Figure
4.16
If the car is moving at constant speed, it is not
accelerating. The forces are balanced.
If the car is
speeding up, the
drive force is
stronger than the
air resistance and
rolling resistance
combined.
If the car is
slowing down,
air resistance &
rolling resistance
are stronger.
124. Review: Forces
Forces come in equal and
opposite pairs.
Objects always push one
one another with equal
force in opposite
directions. (Law of force
pairs)
The less massive object
accelerates more (Law of
motion)
Force on basketball = Force on tennis ball
but in the opposite direction
Tennis ball accelerates (bounces) more
128. Anatomy of the
Equation
Momentum and mass rise together.
For objects traveling at the same speed, the more
massive object has more momentum.
Example: A bullet and a rocket are both traveling at 700
mph. The rocket has more mass, so it also has more
momentum.
p = mv
129. Anatomy of the
Equation
Momentum and velocity rise together.
For objects of the same mass, the faster-moving
object has more momentum.
Example: A running child has more momentum
than her walking twin.
p = mv
131. Momentum can be
transferred
One object can give its momentum to another during an
interaction of a collision.
In the image below, the white cue ball gave its momentum to
the yellow ball.
http://physics.tutorvista.com/momentum/conservation-of-momentum.html
132. Momentum can be carried
through the ground
Thought experiment: Roll on a
chair across the floor.
Observation: You move
forward. You therefore have
momentum.
Observation: You then come to
a stop.
Question: What happened to
your momentum?
https://www.gettyimages.com/detail/photo/mature-woman-sliding-across-office-floor-in-chair-royalty-free-image/EC4919-001
133. Momentum can be carried
through the ground
Answer: The ground
carried the momentum
forward via pressure
waves.
If these waves were
strong enough, we call
them seismic waves, or
Earthquakes. http://www.gauss-centre.eu/gauss-centre/EN/Projects/EnvironmentEnergy/igel-seismic-waves.html?nn=1345670
134. Momentum adds &
subtracts.
If two objects are moving in the same direction, their
momenta add.
Momentum: 1
Total momentum: 2
In this image, each
train engine has 1
unit of momentum to
the right.
Their combined
momentum is the
sum.
Momentum: 1
135. Momentum adds &
subtracts.
If two objects are moving in opposite directions, their
momenta subtract.
Rightward momentum,
1 unit
Total momentum: Zero
Leftward momentum,
1 unit
http://www.stevedennie.com/when-helmets-collide/
136. Law of Conservation of
Momentum
The total momentum of any system
remains unchanged, unless the system is
acted upon by an external force.
In other words, momentum can be transferred from one
object to another, but it can’t be created or destroyed.
137. Equation for Conservation
of Momentum
The change in total momentum equals zero.
(Or: The total momentum of the Universe stays constant.)
Mathematically, the Law of Force Pairs is the same as
Conservation of Momentum.
Type equation here.
p = 0
139. Sound off: Physics
Rocks!
The tendency to keep moving
is called momentum
A moving object has lots of momentum
If it has lots of mass
Or if it’s moving fast.
140. Sound off: Physics
Rocks!
When two objects collide
Whether bouncing or sticking together
The total momentum before
Is the same as after.
Physics rocks!
141. Physics and Human Affairs
Lecture 3 – How Things Move:
Galileo Asks the Right Questions
142. Appetizer:
A Monster in Space
We live in the Milky Way Galaxy,
a spiral-shaped collection of
400 billion stars.
We can’t go outside the Milky
Way to take a picture, but we
have taken pictures of many
other galaxies.
This image is an artist’s
rendition.
This will not be on the exam.
Map of the Milky Way sing data
from WISE spacecraft
https://www.jpl.nasa.gov/news/news.php?release=2010-179
143. Appetizer:
A Monster in Space
Courtesy Max-Planck Institute for Extraterrestrial Physics
Images taken at European Southern Observatory, La Silla, Chile
http://www.youtube.com/watch?v=u_gggKHvfGw
At the center of the Milky
Way Galaxy is a
supermassive black hole, 4
million times as massive as
our Sun.
Many stars orbit this
monster. The innermost
star, named S2, completes
its orbit every 16 years.
This will not be on the exam.
144. Appetizer:
A Monster in Space
This will not be on the exam.
A better visualization of the stars orbiting the black hole.
https://phys.org/news/2013-06-s-star-cluster-galactic-center.html
145. Sound off: Physics
Rocks!
The Metric System isn’t hard.
A meter is close to a yard.
A gram is the mass of a candle wick.
The rest is just a prefix:
146. Sound off: Physics
Rocks!
Kilo means thousand.
Mega means million.
Giga means billion.
Milli means thousandth.
Micro means millionth.
Nano means billionth.
Physics rocks!
147. Lecture 3
Chapter 3
How Things Move:
Galileo asks the
right questions
https://faculty.history.wisc.edu/sommerville/351/351images/galileo.gif
148. 1. Aristotle
Aristotle (4th Century BCE)
developed a set of physical
laws to explain observed
phenomena.
His laws have since been
disproven, but future
scientists would use his
methods to develop the
scientific process.
https://en.wikipedia.org/wiki/Aristotle
150. Aristotle’s Physics
Observations:
If you drop a rock, it falls.
A rock falls faster than a feather.
Water rolls downhill.
Flames leap upward.
The sky exhibits repeatable patterns (day & night,
phases of the Moon, seasons).
If you push a cart and let go, it will come to a stop.
151. Aristotle’s Physics
Hypothesis: Matter can be
divided into 5 elements:
Earth (solid)
Water (liquid)
Air (gas)
Fire (heat)
Aether (the sky)
http://d2r5da613aq50s.cloudfront.net/wp-content/uploads/315358.image0.jpg
152. Aristotle’s Physics
Hypothesis, continued: Motion can be divided into
three types:
Natural motion: The five elements seek their own
place in the Universe.
- Earth seeks to move downward.
- Water seeks it place above Earth.
- Air seeks its place above Water.
- Fire seeks its place above Air.
Violent motion: motion influenced
by humans or animals.
Celestial motion: Motion of the planets, stars, etc.
153. Testing Aristotle’s
Physics
Prediction of Aristotle’s Physics: Heavy objects fall
faster.
Experiment: Book vs. flat sheet of paper.
Result: The book falls faster. Hypothesis supported.
154. Another test of
Aristotle’s Physics
Experiment: Flat sheet of paper vs. crumpled sheet.
Result: Crumpled sheet falls faster, even though the two
have the same mass. Hypothesis contradicted.
156. Galileo
Galileo (1610) recognized the
weaknesses of Aristotle’s
physics, but…
He saw the value in Aristotle’s
observation-based approach.
So…
He further developed
Aristotle’s methods into what
we now call…
The Scientific Process!
https://www.smithsonianmag.com/smart-news/happy-452nd-birthday-galileo-180958148/
157. Galileo’s Scientific
Process
Galileo studied motion using the following tools:
Experiment, or a controlled observation designed to test a
specific hypothesis.
Idealization, or eliminating external influences. For
example, minimizing friction as much as possible.
Limiting the scope of inquiry to answer one question at a
time.
Quantitative methods. Take measurements and do math.
158. Inertia
Hypothesis (Aristotle): A moving object eventually comes to a
stop, due to it natural motion.
Observation (Galileo): If you minimize friction, a ball rolling
on a flat surface will keep rolling.
Aristotle’s hypothesis contradicted.
Hypothesis (Galileo): A moving object has inertia, or a
tendency to keep moving.
159. Inertia
After many experiments, Galileo (and later, Isaac Newton)
would formalize this concept into the Law of Inertia:
An object in motion will stay in motion
until an external force acts on it.
This would eventually be called Newton’s First Law of Motion.
162. Examples
If t represents time, then
Δt means “change in time”,
or elapsed time.
If x represents position,
Δx means “change in
position”, or
straight-line distance
between start and finish.
Clock image: http://i.ebayimg.com/00/s/NDk5WDM4OA==/z/uOsAAMXQVERS-0No/$_3.JPG?set_id=2
Child image: http://www.edmasters.co.in/workshopandseminarsinchennai/flash/index.html
Tape measure image: http://blog.tigerstop.com/accurately-reading-a-tape-measure
Δt
Δx
163. Speed…
Definition
Speed: a change in position over a very small time (Δx / Δt)
Units
Meters per second (m/s)
Kilometers per hour (km/h)
Miles per hour (mph)
164. …as opposed to
velocity
Definition
Velocity: a speed and a direction
A quantity (like velocity) that includes a direction is
called a vector.
Examples
Speed: 60 km/h
Velocity: 60 km/h westbound
165. Acceleration
Acceleration: a change in velocity over time
(a = Δv / Δt)
Units: meters per second squared (m/s2), feet per
second squared (ft/s2), etc.
166. Acceleration
Example
A bicyclist is beginning her ride.
She begins at rest (zero velocity).
After 1 second (Δt = 1 s), her speed
increased from 0 to 5
meters per second (Δv = 5 m/s).
Therefore, her speed increased by 5 meters per second …
per second.
Acceleration is 5 meters per second squared (a = 5 m/s2)
http://www.thai-swiss-cycling.ch/site/en/
167. How to accelerate
Acceleration is a change in velocity over time.
Velocity includes both speed and direction.
Therefore, there two ways to accelerate:
Change speed (speed up or slow down)
Change direction (turn a corner, or move on a curved
path)
168. Direction of
acceleration
Acceleration, like velocity, is a vector, meaning:
It has direction.
• To speed up is to accelerate forward
• To slow down is to accelerate backward
• To turn a corner is to accelerate inward, toward the
center of the curve. This is called centripetal
acceleration.
169. How NOT to accelerate
Two ways NOT to accelerate:
Stay still
Move in a straight line with constant speed
170. Let’s play a game:
Is it accelerating?
Is each object accelerating, or not?
An asteroid in outer space, moving in a straight line with
constant speed.
It’s not changing speed or direction, so NO.
A moon in a circular orbit around Jupiter, moving at
constant speed.
It’s moving on a curve path, so its direction is changing.
YES. It is accelerating toward the center of the orbit
(toward Jupiter).
171. Let’s play a game:
Is it accelerating?
Is each object accelerating, or not?
A comet gaining speed as it falls toward the Sun.
Its speed is changing, so YES. Since it is speeding up,
it is accelerating in the forward direction.
A probe losing speed as it breaks Earth’s gravity.
Its speed is changing, so YES. Since it is slowing
down, it is accelerating in the backward direction.
172. 3. Freefall
Definition of Freefall: falling with no
influence except gravity. In other words,
falling without air resistance.
Hypothesis (Galileo): Objects in freefall
gain speed as they fall.
174. Freefall
Galileo’s Analysis:
As the slope gets
steeper, the ball moves
closer and closer to a
state of freefall.
He used math to
extrapolate the results
from rolling to freefall. https://www.youtube.com/watch?v=tUmVqgBp06s
175. Galileo’s Law of Falling
Aristotle said heavy objects fall faster than light objects.
Using both logical arguments and experiments, Galileo
struck down Aristotle’s physics.
He came up with a new law of falling:
If air resistance is negligible, all objects
fall in the same way.
176. Galileo’s law of falling,
in action
5.5 minute video
https://aetn.pbslearningmedia.org/resource/nvmm-
math-fallingbodies/galileos-falling-
bodies/#.WmO2kUtG3fA
177. Galileo’s law of
falling, in action
This image shows the same basketball
at various points in its fall.
The positions are at regular time
intervals. The numbers represent the
distance the basketball has fallen.
Note that the balls covers more and
more distance as it falls.
In other words, it speeds up as it falls.
https://en.wikipedia.org/wiki/Equations_for_a_falling_body
178. Acceleration due to
gravity
An object in freefall accelerates
toward Earth at about 10 m/s2.
After 1 second, your speed is 10 m/s. After 2 seconds, your speed
has doubled. After 3 seconds, your speed has tripled.
Remember, freefall mean no air resistance.
Note: this acceleration is specific to Earth. On the Moon, the
acceleration due to gravity is about 1/6 as much!
182. Appetizer
This will not be on the exam.
The
Andromeda
Galaxy is on
course to
collide with
our Milky
Way.
https://www.youtube.com/watch?v=4disyKG7XtU
Credit: Visualization: NASA; ESA; and F. Summers, STScI | Simulation: NASA; ESA; G. Besla,
Columbia University; and R. van der Marel, STScI)
183. This is what the collision might look like from Earth.
https://www.youtube.com/watch?v=k1Dm6taJRZQ
184. Sound off: Physics
Rocks!
A falling ball
Doesn’t fall
at a constant rate.
If it’s in freefall
It’ll accelerate.
185. Sound off: Physics
Rocks!
Air resistance
Makes a feather fall slow.
Without resistance,
It drops like a stone.
Physics rocks!
186. Lecture 4a
Chapter 4
Why Things
Move as
They Do,
Pt. 1
https://www.tes.com/lessons/YxVM6sbRefnaNg/motion-by-kayla-and-brenden
187. Newton Discovers
Physics
Isaac Newton (1687) built on
Galileo’s work.
He also developed a new branch
of mathematics – Calculus – in
order to study motion.
The mathematician Leibniz also
developed calculus.
Newton and Leibniz didn’t like
each other.
Portrait of Newton
by Godfrey Kneller
https://en.wikipedia.org/wiki/Isaac_Newton
188. Concepts for
Newtonian Physics
Velocity is a speed and a direction.
Acceleration is a change in velocity. In other
words, a change in speed or direction.
Mass is the amount of matter in an object.
Force: A force is a push or a pull.
189. Principles in Newtonian
Physics
Law of Inertia: An object in motion will stay in
motion unless acted on by a force.
The Law of Inertia was discovered by Galileo
and further studied by Newton.
It is sometimes called Newton’s 1st Law.
190. Principles in Newtonian
Physics
Law of Falling: All objects in freefall (with no air
resistance) fall with the same acceleration.
The Law of Falling was discovered by Galileo
and further studied by Newton.
191. Principles in Newtonian
Physics
Principles discovered by Newton:
The Law of Motion (sometimes called
Newton’s 2nd Law).
The Law of Force Pairs (sometimes called
Newton’s 3rd Law).
The Law of Gravity.
192. 4.1 Force: Why Things
Accelerate
A force is a push or a pull.
Equivalently, a force in an influence that can
cause an acceleration.
pic
https://byjus.com/physics/force-push-and-pull/
193. Examples of forces
Kicking a chair
Pulling a rope
Friction
Air resistance
Gravity
…and many more!
http://moziru.com/explore/Wars%20clipart%20gravitational%20force/
194. Forces are external.
An object cannot cause itself
to accelerate. It must push
or pull something else.
Example: When a car
accelerates, it pushes off the
ground. It is the ground
that provides the force.
Example: When a rocket
accelerates in space, it is
pushing off its exhaust. The
exhaust provides the force.
Saturn V Rocket
https://www.space.com/38719-saturn-v-rocket-50th-launch-anniversary.html
195. Unit of Force:
the Newton
In the American unit system, the pound (lb) is
a unit of force.
In the Metric System, the Newton (N) is a
unit of force.
The Newton, like the pound, is also a unit of
weight.
The reason … your weight is the force with
which gravity pulls you.
196. Net force
A force is an influence that can cause
acceleration.
A net force is an influence that does cause
acceleration.
So what exactly is a net force?
A net force is an imbalance of forces acting
on an object.
197. Example: No Net Force
Tug of war:
In this image,
the left-pulling
forces are balanced
by the right-pulling
forces.
There are plenty of
forces on the rope, but they are balanced.
There is no net force on the rope, so…
The rope does not accelerate.
https://www.shutterstock.com/video/search/tug-of-war
198. Example: Yes Net Force
In this image, the right-pulling forces are stronger.
There is an imbalance of forces to the right.
In other words, the rope feels a net force to the
right.
Therefore, the rope accelerates to the right.
http://clipart-library.com/cartoon-tug-of-war.html
200. 4.2 Connecting Force
and Acceleration
Mass
Mass is the amount of matter in an object.
Mass is like weight, except weight depends on
gravity and mass does not.
For example: the Moon has less gravity than Earth.
So, you’d weigh less on the Moon, but you’d have the
same mass.
201. Why mass matters
The more massive an object, the harder it is to
accelerate.
In other words, the more massive an object, the less
it will respond to a net force.
202. Thought Experiment:
Marble vs. Bowling Ball
Suppose I flick my finger on a
marble.
In that case, I have applied a
force.
The marble accelerates.
Now, apply the same force (a
finger flick) to a bowling ball.
The bowling ball is more
massive than the marble, so it
will accelerate less.
http://www.kingofalltechnology.com/marblegames.htm
https://www.123rf.com/photo_35563099_business-man-finger-flick-something.html
https://www.shutterstock.com/image-vector/red-bowling-ball-isolated-on-transparent-548894737
203. Review: units of mass
The gram (g) is a unit of
mass. A calcium
supplement has a mass
of about 1 gram.
A kilogram (kg) is 1,000
grams. A textbook has a
mass of about 1 kg.
1 g of acetaminophen
https://www.drugs.com/acetaminophen.html
About 1 kg of paper
204. Pound vs. Kilogram
The pound is a unit of
force (or, equivalently,
weight).
On Earth, a 1-kg object
weighs about 2.2
pounds.
On the Moon, the same
1-kg object only weighs
0.4 pounds (about 6
ounces). http://nineplanets.org/luna.html
205. 4.3. Newton’s Law of
Motion
Newton’s Law of Motion is also known as Newton’s 2nd
Law. It states:
If an object feels a net force, it will accelerate.
The stronger the net force, the more the
acceleration.
The more massive the object, the less the
acceleration.
206. Mathematical form of
Newton’s Law of Motion
Newton’s Law of Motion is often stated in the form of
an equation:
F = ma
Net force equals mass times acceleration
207. How things accelerate
In order to understand the
equation, let’s use algebra
to rearrange it.
F = ma
208. How things accelerate
Let’s solve for acceleration.
To get acceleration by
itself, divide both sides by
mass.
F = ma
m m
212. How things accelerate
What does this equation
mean? There are two
things to notice:
1. Net force is in the top of
the fraction (the
numerator).
F = a
m
213. How things accelerate
What does this equation
mean? There are two
things to notice:
1. Net force is in the top of
the fraction (the
numerator).
That means F and a rise and
fall together.
F = a
m
215. How things accelerate
There second thing to
notice:
2. Mass is in the bottom of
the fraction (the
denominator).
F = a
m
216. How things accelerate
There second thing to
notice:
2. Mass is in the bottom of
the fraction (the
denominator).
That means m and a rise
inversely to each other.
F = a
m
217. How things accelerate
F = a
m
In other words…
More massive objects
are harder to accelerate
218. Example 1: Same force,
difference masses
A bowling ball is more massive than a soccer ball.
So, given the same force (kick), the soccer ball will gain
speed more quickly (accelerate more).
219. Example 2: Difference
forces, same mass
Federer vs. Puppy.
Swiss professional
tennis player Roger
Federer can apply a
stronger force than a
puppy.
Therefore, he can make
a tennis ball accelerate
more.
https://en.wikipedia.org/wiki/Roger_Federer
https://www.youtube.com/watch?v=AyZdxgEuXmo
Federer puppy
221. Sound off: Physics
Rocks!
A stronger net force,
Means more acceleration, of course
But an object with more mass
Will accelerate less.
Physics rocks!
223. PHA Appetizer:
Astronomy Picture of the Day
Got 5 minutes to learn something about space while
looking at a pretty picture?
Check out Astronomy Picture of the Day! You’ll find:
• A different image or short video every day
• Captions written by real astronomers
• Searchable archives
Go to https://apod.nasa.gov/
This will not be on the exam.
224. Sound off: Physics
Rocks!
Science is a game of guess and check.
Hypothesis is the guess;
observation is the check.
No matter how much you think you know
If the data says no,
Let your hypothesis go.
225. Lecture 2
Chapter 2 - Atoms:
The Nature of Things
https://www.thoughtco.com/interesting-facts-about-atoms-603817
226. I. Units
Q: How many feet are in a mile?
A: 5,280
Q: How many inches are in a mile?
A: Uh…multiply that by 12.
Q: How many teaspoons are in a gallon?
A: Please stop.
Imperial units suck!
227. An Easier Way: The
Metric System
The Metric System is a set of units that is designed to
be easy to use.
The Metric System is used by:
• Scientists, and
• Practically everyone who isn’t American.
You only need to know a few basic units and a few
basic prefixes.
228. The gram
The gram is a unit of mass.
Mass is like weight, except it doesn’t depend on
gravity.
The Moon has less gravity than Earth, so you’d weigh
less on the moon, but your mass would be the same.
In other words, you’d have fewer pounds on the
Moon, but the same number of grams.
229. How massive is a gram?
A large pill has a mass of about 1 gram.
http://www.telegraph.co.uk/health-fitness/body/birth-control-can-you-really-die-from-taking-the-contraceptive-p/
230. The Meter
The meter is a unit of length.
It is about 39.4 inches, or slightly more than a yard.
https://www.flinnsci.com/meter-stick-hardwood-double-sided-metric/ap6012/
231. Prefixes
In the Metric System, you can make a unit larger
smaller by adding a prefix.
For example, kilo means 1,000. Therefore, a kilogram
is 1,000 grams. (your textbook has a mass of about 1
kilogram)
As another example, milli means 1/1000. So a
millimeter is 1/1000 of a meter, or 0.001 meters.
232. A few of the important
prefixes
Prefix Meaning
kilo thousand
mega million
giga billion
Prefix Meaning
centi hundredth
milli thousandth
micro millionth
nano billionth
233. Example Units, with
prefixes
Length
Mass
Unit Symbol Scale
milligram mg Strand of hair
gram g Pill
kilogram kg Book
Unit Symbol Scale
millimeter mm Thickness of
fingernail
centimeter cm Thickness of
finger
meter m Length of arm
kilometer km Distance
across UARK
campus
235. Atomic Theory of
Matter
Matter is made of atoms.
But what are atoms?
The definition has changed over time! But for now,
we’ll say:
Atoms are tiny particles, too small to be seen.
236. History of Atoms:
Democritus
Observation: We can smell
bread.
Hypothesis: Bread must
be made of tiny particles
that break off and enter
our nose.
These particles, called
atoms, are indivisible
(they can’t be broken
apart)
- Democritus, 5th Century BCE
http://www.healthguidance.org/entry/16642/1/The-Smell-of-Bread-Baking-Makes-Us-Kinder.html
237. Dalton makes a
hypothesis…
Observation: When hydrogen
& oxygen combine to form
water, they always react in an
8:1 ratio.
For example, 8 kg of oxygen
combines with 1 kg hydrogen.
Hypothesis: Hydrogen and
oxygen are atoms, and oxygen
is 8 times as heavy as
hydrogen.
- John Dalton, 1803
http://chemed.chem.purdue.edu/genchem/history/dalton.html
238. …and gets it wrong.
Note: We now
know that an
oxygen atom is 16
times as heavy as a
hydrogen atom…
But it takes 2
hydrogens to make
water (H2O)…
Hence, the 8:1 ratio.
239. Further experiments
get it right
Dalton provided the first
experimental evidence
for the atomic theory.
But he got the details
wrong!
The next generation of
scientists, building on his
results, got it right.
Science isn’t perfect. It
just gets better with age. John Dalton
https://www.biography.com/.image/ar_1:1%2Cc_fill%2Ccs_srgb%2Cg_face%2Cq_80%2Cw_300/MTE1ODA0OTcxNTk5OTU1NDY5/john-dalton-9265201-1-402.jpg
240. Another hypothesis:
Atoms bombard dust
Observation: Tiny dust
grains jiggle in water.
Hypothesis: The grains
are being bombarded
by atoms (or molecules)
from every angle.
- Robert Brown, 1827
Note: we now call this
behavior Brownian
motion.
Brownian motion
https://scottbembenek.com/einsteins-paper-on-brownian-motion/
241. Math confirms the
hypothesis
Analysis: Albert
Einstein (1905) used
Brown’s hypothesis to
compute how dust
would grains would
disperse in water.
Einstein’s
computations were
consistent with
measurements.
Brown’s atomic
hypothesis was
supported. https://briankoberlein.com/2015/05/05/shake-rattle-and-roll/
242. Another test of Atomic
Theory
Hypothesis: If Brownian
motion is caused by atoms
(or molecules), more
massive dust grains should
be knocked around less.
Experiment: Jean-Baptiste
Perrin (1909) studied dust
grains of different masses
jiggling in water.
Brown’s hypothesis (and
Einstein’s computation)
were confirmed.
http://www.sciencephoto.com/media/611608/view
243. Math strengthens
Atomic Theory
Observation: Air contains more
oxygen at low altitudes.
Dust grains in water also
behave this way. More grains
end up near the bottom, and
fewer near the top.
Analysis: By comparing the
behaviors of dust grains in
water and oxygen in the
atmosphere, Perrin was able to
compute the mass of an
oxygen molecule!
245. Things that can be explained by
Atomic Theory: Phases of water
Phases of water
Solid (ice) Liquid Gas (vapor)
https://www.sciencelearn.org.nz/resources/607-solids-liquids-and-gases
246. Things that can be explained by
Atomic Theory: Temperature
Temperature
Temperature is
the vibration of
atoms.
The harder the
atoms vibrate,
the hotter the
temperature.
http://hop.concord.org/h1/phys/h1pf.html
247. So Atoms are real,
but…
Conclusion: We get it. Atoms are
real. But seriously, WTF are they???
248. Atomic Model: Greeks
Greek Model
(5th Century BCE)
Atoms are tiny, indivisible
particles. Like
microscopic marbles.
249. Discovery of the
Electron
Planetary Model
J.J. Thompson (1897)
discovers the electron,
which he calculated was
lighter than an atom.
Did that means atoms
aren’t indivisible after all?
Yup…
Sir J.J. Thompson
https://www.britannica.com/biography/J-J-Thomson
250. Planetary Atomic Model
Planetary Model
Ernest Rutherford
(1911) discovers the
nucleus, the dense spot
in the center of the
atom where most of
the mass is.
Hypothesis: Electrons
orbit the nucleus like
planets orbiting the
Sun.
https://www.quora.com/Electricity-is-the-flow-of-electrons-Where-do-these-electrons-come-from-How-are-they-put-into-circulation-and-what-happens-to-them
251. Rutherford’s model
contradicts experiment
Planetary Model
Observation: An
accelerating charged
particle radiates away its
energy.
Analysis: Orbiting electrons
should therefore radiate
away their energy and spiral
into the nucleus.
If Rutherford’s model is
correct, atoms would be
unstable and we wouldn’t
exist. This cannot be!
https://www.quora.com/Why-is-Rutherfords-model-wrong
252. Bohr Model: Energy
Levels
Bohr Model (1913)
Hypothesis: Electrons orbit
the nucleus like planets
orbiting the Sun, but with
discrete energy levels.
In others, electrons can only
orbit at certain distances.
They cannot spiral inward.
This model works pretty well
for hydrogen, but not for
atoms with more than one
electron.
https://www.askiitians.com/iit-jee-structure-of-atom-and-nucleus/bohr-model/
253. Quantum Model
Quantum Model (1920’s)
Quantum experiments
rocked everyone’s world.
We’ll get to them later in
the semester.
For now, we’ll just focus
on the results.
http://www.theonering.net/torwp/2012/09/15/61799-upcoming-hobbit-trailer-to-feature-epic-music-from-quantum/quantum-meta-cover/
254. Electron Clouds
Quantum Model (1920’s)
Experiment: Electrons do
orbit in discrete energy
levels, but they are not like
planets orbiting the Sun.
Rather, electrons are waves
that hang around the
nucleus like a cloud.
We sometimes refer to
them as an electron cloud. Some possible shapes of the
Hydrogen electron cloud
https://en.wikipedia.org/wiki/Quantum_mechanics
255. Definitions: Atoms
A proton is a tiny, compact particle with positive electrical
charge.
A neutron is a tiny, compact particle with zero charge.
An atom consists of:
• A tiny compact nucleus that consists of protons and
sometimes neutrons
• Usually, one or more negatively charged electrons
surrounding the nucleus like a wave or cloud
A molecule is two or more atoms electrically bound
together
256. Review: Atomic Models
Evolution of Atomic Theory
Greek Model (5th Century BCE): Atoms are tiny, indivisible
particles.
Planetary Model (1897 – 1920’s): Electrons orbit the nucleus
like planets orbit the Sun.
Bohr Model (Not correct but sometimes useful): Electrons
orbit the nucleus like planets, but in discrete energy levels.
Quantum Model (1920’s – Present): Electrons are waves
(clouds) that surrounds the nucleus with discrete energy
levels.
257. Sound off: Physics
Rocks!
The Greeks thought the atom
Was as small as you could get
But in 1897
The electron was smaller yet.
258. Sound off: Physics
Rocks!
By the 1920’s
Electrons were waves.
Science isn’t perfect;
It just gets better with age.
Physics rocks!
260. Contacting the Instructor
Instructor: Dr.
Deanna Shields
(she/her)
Contact:
DShields@uark.edu
Office Hours: 11 am
– 12pm, Monday
through Thursday or
by appointment
Office: Physics
Building (PHYS) 237
Physics Building
261. PHA Lecture & Lab
This class, PHYS 1023, is lecture only.
The associated lab, PHYS 1021L, is a separate course
with a separate grade.
Lab questions should be directed to your lab
instructor.
262. Let’s look at the
syllabus.
Go to Blackboard and check it out.
264. The Scientific Process
The scientific process is a formal game of guess and check. It
is an interplay among:
Imagination to formulate hypothesis
Prediction based on hypothesis
Observation or experiment to test hypothesis
Well-reasoned analysis
Objective thought (this takes some training)
Math (sorry)
The Scientific Process
The scientific process is a formal game of guess and check. It
is an interplay among:
Imagination to formulate hypothesis
Prediction based on hypothesis
Observation or experiment to test hypothesis
Well-reasoned analysis
Objective thought (this takes some training)
Math (sorry)
266. Example:
Is the world flat?
Hypothesis: The
world is flat.
Prediction: If the
world is flat, then I
should be able to
see all 7 continents
(out to the edge of
the world) from an
airplane.
Nothing would ever
be below the
horizon.
Image: http://static5.businessinsider.com/image/5a207613f914c347018b71c8/neil-degrasse-tyson-called-out-flat-earthers--heres-what-would-happen-if-the-earth-were-actually-flat.jpg
267. Evidence: visibility of
Continents
Observation:
Board an airplane on
the West Coast.
Look westward onto
the Pacific.
Do I see Japan?
No, Japan is below the
horizon. The round
Earth blocks our view.
https://southsudanmedicaljournal.files.wordpress.com/2012/01/img_0430.jpg
269. Observation: Alpha
Centauri
Alpha Centauri is deep in the
southern sky. It can only be seen
by people in the Tropics and the
Southern hemisphere.
It can never be seen from
Arkansas because it never rises.
The round Earth blocks our view.
Alpha Centauri
270. Example:
Is the world flat?
Objective thought:
I like the idea that the world is
flat.
However, observation contradicts
my hypothesis.
I must therefore abandon my
hypothesis. (That is hard!)
Conclusion: The world is not flat.
https://cdn.pixabay.com/photo/2014/02/23/09/17/thinking-272677_960_720.jpg
273. Hypothesis 1: the
Heliocentric model
How do we explain
retrograde motion?
Hypothesis 1: Earth & Mars
are both orbiting the Sun.
Earth is moving faster.
Retrograde motion
happens when Earth
“passes” mars in its orbit.
- Aristarchus, 2nd Century
BCE http://pages.uoregon.edu/soper/Orbits/marsorbit.gif
274. Hypothesis 2: the
Geocentric model
How do we explain
retrograde motion?
Hypothesis 2: Earth is
standing still, and Mars is
orbiting earth in epicycles
(loop-the-loops)
- Ptolemy, 2nd Century CE
https://physics.weber.edu/schroeder/ua/Epicycle.png
275. Objective thought:
Stellar parallax
Objective thought: If Earth
is orbiting the Sun, our
point of view should be
shifting as the Earth
moves.
The stars should therefore
appear move in tiny loops,
once a year.
This is called stellar
parallax.
277. But wait!
Objective thought:
If the stars are really, really
far away (say, trillions of
miles or more), stellar
parallax would be a small
effect.
We wouldn’t be able to see it
with the naked eye.
Maybe the Earth really does
around the Sun?
- Copernicus, 1543
278. Designing a test
If Venus is closer to the Sun than Earth, and if both go around the Sun, then
Venus should show a full range of phases, like the Moon.
But if Ptolemy’s model is correct, Venus should always be in a crescent
phase.
- Galileo, 1610
279. Galileo’s Telescope
Observation
Observation:
1610: Galileo had a brand new thing
called a telescope!
He looked at the phases of Venus. It
turns out Venus has the full range of
phase.
Therefore, Venus goes around the
Sun.
This is evidence for the heliocentric
model.
280. What about stellar
parallax?
Objective thought: If Earth
goes around the Sun, we
should see stellar parallax.
Observation: Stellar parallax
was first observed by Friedrich
Bessel in 1838.
Gaia Spacecraft has now
measured parallax for 1 billion
stars.
281. Conclusion
Conclusion: Earth is
not the center of the
Universe.
Rather, it is a planet.
Like the other
planets in our Solar
System, Earth orbits
the Sun.
282. Sound off: Physics
Rocks!
Science is a game of guess and check.
Hypothesis is the guess,
Observation the check.
No matter what you think you know
If the data says no,
Let your hypothesis go.
Physics rocks!
