Transcript Class 5 S11 (Ch 3b and 4a)Jan. 25

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Ch 3 (Histrory of Astronomy)
1.
2.
Ancient Roots of Science (Archeoastronomy)
Astronomy of Greece: good records & more rational approach
a. Thales and Pithagoras: roots of science
b. Plato and Aristotle: Geocentric universe
c. Ptolemy: mathematical models of geocentric views
3.
4.
The Copernican revolution
The Nature of Science
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3.3 The Copernican Revolution
Histrory of Astronomy PART II: “The Church Strikes Back”
1.
2.
3.
4.
5.
Problems with Ptolemy’s models
Copernicus: publishes heliocentric model and dies
Tycho observes planetary motions and dies
Kepler: uses Tycho’s observations & writes 3 laws (on
pages 67-69 of book):
a. Each planet moves in ellipse with the Sun at one focus.
b. The line between the Sun ……. (faster near Sun and vice versa)
c. p2 = a3
Galileo:
a.Uses telescope to discover moons of Jupiter, study
sunspots and phases of Venus
b.Supports Copernicus’ theory & gets in trouble with
Church
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Copernicus’ Heliocentric Model
•With
better observations more problems
with the Ptolemaic model became
apparent…..and created more complex
fixes
• Copernucus’ solution was to put the Sun
at the center.
•The math was much simpler in
Copernucus’ solution
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Copernicus’
Model
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Ptolemy’s
Model
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Copernicus’ Heliocentric Model
•Sun
is at center
•Earth orbits like any other planet
•Inferior planet orbits are smaller
•Apparent retrograde motion occurs when
we “lap” Mars & the other planets
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Occam’s Razor
• Among competing theories, the best
theory is the simplest theory-that is,
the one with the fewest assumptions.
• Heliocentric model is much simpler than
Ptolemy’s model and more accurate at
predicting how the planets move in the sky
• Father of observational
astronomy.
• Danish nobleman and
Scientist. Lost his nose in a
Tycho Brahe
duel….
• He had a young assistant,
Johannes Kepler
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Johannes Kepler
• German astronomer and
mathematician
• Used Tycho’s observations to
write 3 laws of planetary
motion
• He was deeply religious and actually
strongly believed in the predictive power
of astrology
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Kepler’s Laws (on page 67-69 of book)
1 Each planet’s orbit around the Sun is an
ellipse, with the Sun at one focus.
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Kepler’s Laws
2 A planet moves along its orbit with a speed that
changes in such a way that a line from the planet to
the Sun sweeps out equal areas in equal intervals of
time. (the closer to the Sun, the faster it moves)
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Kepler’s Laws
3 The ratio of the cube of a planet’s average distance
from the Sun to the square of its orbital period is
the same for each planet.
3
2
a /P = 1
3
a =P
2
a in AU
P in years
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• Kepler first tried to match Tycho’s
observations with circular orbits
• But an 8 arcminute discrepancy led
him eventually to ellipses…
Johannes Kepler
(1571-1630)
If I had believed that we could ignore
these eight minutes [of arc], I would
have patched up my hypothesis
accordingly. But, since it was not
permissible to ignore, those eight
minutes pointed the road to a
complete reformation in astronomy.
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Galileo Galilei (1564-1642)
• First man to point a
telescope at the sky
• wanted to connect
physics on Earth with
the heavens
• Wrote the book
“Dialogue Concerning
the Two Chief World
Systems” .
This book got him in trouble with the Church!
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Galileo’s Observations
• Galileo discovered
that Jupiter had four
moons of its own.
• Jupiter was the
center of its own
system.
• Heavenly bodies
existed which did not
orbit the earth.
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Galileo’s observation of the phases of Venus was the
final evidence which buried the geocentric model.
Geocentric
No gibbous or full phases!
Heliocentric
All phases are seen!
Galileo© 2005
observed
all phases!
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The Catholic Church ordered
Galileo to recant his claim
that Earth orbits the Sun in
1633
His book on the subject was
removed from the Church’s
index of banned books in
1824
Galileo Galilei
Galileo was formally
vindicated (pardoned) by the
Church in 1992
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What have we learned?
• How did Copernicus, Tycho and Kepler challenge
the Earth-centered idea?
• Copernicus created a sun-centered model; Tycho
provided the data needed to improve this model;
Kepler found a model that fit Tycho’s data.
• What was Galileo’s role in the Copernican
revolution?
• His experiments and observations overcame the
remaining objections to the Sun-centered solar
system
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Ch 3 (Histrory of Astronomy) “Does the World Turn?”
1.
2.
Archeoastronomy
Astronomy of Greece: good records & more rational approach
a. Thales and Pithagoras: roots of science
b. Plato and Aristotle: Geocentric universe
c. Ptolemy: mathematical models of geocentric views
3.
The Copernican revolution “The Church Strikes back”
4.
The Nature of Science
3.4 The Nature of Science
Our goals for learning:
How can we distinguish science from
nonscience?
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Science and Pseudoscience
(not exactly like the book)
I.
Science:
1.
2.
3.
II.
Based on observations and theory
Open to criticism and constant challenge in light of
new evidence
Not perfect (human), not always right, but most
successful discipline at predicting the way nature
works.
Pseudoscience
1.
2.
3.
Generally holds “absolute truth”
Considers only part of evidence available
Tends to play on emotions and fears instead of logic
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Hallmarks of Good Science
• Science seeks explanations that rely solely on natural
causes.
• Science progresses by creating and testing models of
nature that explain observations as simply as possible:
Occam’s Razor
• A scientific model must make testable predictions that
could force us to revise or abandon the model.
• Scientific Theory: a model which survives repeated
testing
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Astrology (not in book)





Not a science
Empirical Discipline (no theoretical explanation)
Easily abused by charlatans
Big problem: astrologers do not agree on what
any given celestial configuration means
In other words, who will you believe if one
astrologer tells you this is a great time to travel
and another tells you to stay home?
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Chapter 4
Making Sense of the Universe:
Understanding Motion, Energy, and Gravity
“If I have seen farther than others, it is
because I have stood on the shoulders of
giants.” — Sir Isaac Newton (1642 – 1727)
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Outline of Ch 4 Motion and Gravity
(soap opera’s final episode)
4.1 and 4.2 Describing Motion, Newton and Galileo
Speed, velocity and acceleration (skip momentum)
Galileo’s experiments with falling objects:
g = 9.8 m/sec2
Objects fall together
Inertia (motion in absence of force)
Newton’s Laws:
1.
2.
3 laws of motion: a. Inertia b. F=ma c. Action = Reaction
Gravitation: F= GM1M2/R2 (Inverse-square law)
4.3 (Thermal Energy only ignore the rest of 4.3)
4.4 The force of Gravity

The Strength of Gravity
■ Newton and Kepler

Orbits: 1. Closed: circles (circular velocity) & ellipses (v > v c)
2. Open: parabolas and hyperbolas (escape velocity, v > v e)

Tides: Lunar and Solar
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4.1 Describing Motion
• Our goals for learning:
• How do we describe motion?
• How is mass different from weight?
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How do we describe motion?
Precise definitions to describe motion:
• speed: rate at which object moves

speed = distance units of m

s

time

example: speed of 10 m/s
• velocity: speed and direction
example: 10 m/s, due east
• acceleration: any change in velocity
units of speed/time (m/s2)
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•
The Acceleration of Gravity (some not in
book)
Galileo was one of the
giants on who’s shoulders
Newton stood. Galileo’s
experiments with falling
objects showed that:
• Objects fall together (not
counting friction of air
resistance).
• On Earth, g ≈ 10 m/s2:
speed increases 10 m/s with
each second of falling (g =
9.8 m/s2).
• Inertia (motion in absence
of force: uniform rectilinear
or at rest)
• g = 9.8 m/sec2
• Objects fall together
• Inertia (motion in
absence of force)
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The Acceleration of Gravity (g)
• Galileo showed that
g is the same for all
falling objects,
regardless of their
mass.
Apollo 15 demonstration
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Question:
Is there a net force? Y/N
1.
2.
3.
4.
A car coming to a stop.
A bus speeding up.
A bicycle going around a curve.
A moon orbiting Jupiter.
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Is there a net force? Y/N
1.
2.
3.
4.
A car coming to a stop. Y
A bus speeding up. Y
A bicycle going around a curve. Y
A moon orbiting Jupiter. Y
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How is mass different from weight?
• mass – the amount of matter in an object
• weight – the force that acts upon an object
You are weightless
in free-fall!
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Question
On the Moon:
A.
B.
C.
D.
My weight is the same, my mass is less.
My weight is less, my mass is the same.
My weight is more, my mass is the same.
My weight is more, my mass is less.
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Why are astronauts weightless in space?
• There IS gravity in space…
• weightlessness is due to a constant state of free-fall:
• Ask me about
another way to
explain this
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What have we learned?
•How do we describe motion?
•Speed = distance/time
•Speed + direction => velocity (v)
•Change in velocity => acceleration (a)
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What have we learned?
• How is mass different from
weight?
• Mass = quantity of matter
• Weight = force acting on mass
• Objects are weightless when in
free-fall
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What have we learned?
• Galileo’s experiments with
falling objects:
• On Earth g = 9.8 m/sec2
(m/sec/sec)
• Objects fall together
• Inertia (motion in absence of
force)
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4.2 Newton’s Laws of Motion
Our goals for learning:
• How did Newton change our view of the
universe?
• What are Newton’s three laws of motion?
• What is Newton’s laws of gravitation?
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How did Newton change our view of the Universe?
• Realized the same physical
laws that operate on Earth
also operate in the heavens
 one universe
• Discovered 3 laws of
motion and law of
gravitation
• Much more: experiments
with light; first reflecting
telescope, calculus…
Sir Isaac Newton
(1642-1727)
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What are Newton’s three laws of motion?
Newton’s first law of motion: An object moves at
constant velocity unless a net force acts to change its
speed or direction (this he adopted from Galileo).
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Newton’s second law of motion:
Force = mass  acceleration (F= ma)
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Newton’s third law of motion:
For every force, there is always an equal and opposite
reaction force (action = reaction).
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The Universal Law of Gravitation
1. Every mass attracts every other mass.
2. Attraction is directly proportional to the product of
their masses.
3. Attraction is inversely proportional to the square of
the distance between their centers..
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Question:
Is the force the Earth exerts on you larger, smaller, or
the same as the force you exert on it?
A. Earth exerts a larger force on you.
B. I exert a larger force on Earth.
C. Earth and I exert equal and opposite forces on
each other.
D. There is no force between Earth and any object
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Question:
A compact car and a Mack truck have a head-on
collision. Are the following true or false?
1. The force of the car on the truck is equal and
opposite to the force of the truck on the car.
2. The change of velocity (acceleration) of the car is
the same as the change of velocity of the truck.
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Question:
A compact car and a Mack truck have a head-on
collision. Are the following true or false?
1. The force of the car on the truck is equal and
opposite to the force of the truck on the car.
2. The change of velocity of the car is the same as
the change of velocity of the truck.
(remember F = ma, if “F” is the same and the
masses are very different then “a”, which is the
change in velocity must also be very different)
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What have we learned?
•
How did Newton change our view of the universe?
•
•
•
He discovered laws of motion & gravitation.
He realized these same laws of physics were identical in the
universe and on Earth.
What are Newton’s Three Laws of Motion?
1)
2)
3)
Object moves at constant velocity if no net force is acting.
Force = mass  acceleration
For every force there is an equal and opposite reaction force.
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4.3 Ignore all except Thermal Energy
• Relation temperature  motion of atoms :
• The higher the temperature the faster the atoms in a
substance will be moving
• As atoms collide the electrons collide and their motion is
disturbed
• When the motion of electrons gets disturbed they produce
photons
• The higher the temperature, the more collisions, the more
photons (more about this in Ch. 5)
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Temperature Scales
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Outline of Ch 4 Motion and Gravity
(soap opera’s final episode)
4.1 and 4.2 Describing Motion, Newton and Galileo
Speed, velocity and acceleration (skip momentum)
Galileo’s experiments with falling objects:
g = 9.8 m/sec2
Objects fall together
Inertia (motion in absence of force)
Newton’s Laws:
1.
2.
3 laws of motion: a. Inertia b. F=ma c. Action = Reaction
Gravitation: F= GM1M2/R2 (Inverse-square law)
4.3 (Thermal Energy only)
4.4 The force of Gravity


The Strength of Gravity
■ Newton and Kepler
Orbits: 1. Closed: circles (circular velocity) & ellipses (v > v c)
2. Open: parabolas and hyperbolas (escape velocity, v > v e)

Tides: Lunar and Solar
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4.4 The Force of Gravity
Our goals for learning:
•What determines the strength of gravity?
•How does Newton’s law of gravity extend
Kepler’s laws?
•How do gravity and energy together allow us
to understand orbits?
•How does gravity cause tides?
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What determines the strength of gravity?
The Universal Law of Gravitation
1. Every mass attracts every other mass.
2. Attraction is directly proportional to the product of
their masses.
3. Attraction is inversely proportional to the square of
the distance between their centers..
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How does Newton’s law of gravity extend Kepler’s laws?
(some not in book)
• Ellipses are not the only
orbital paths. Orbits can be:
bound
• Circle (v = vc)
• Ellipse (v > vc)
unbound
• Parabola (v = ve)
• Hyperbola (v > ve)
• Circular and Escape
velocities (vc and ve)
 vc = GM/R
 ve = 2GM/R
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circular and