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Aritstotle: 384-322 BC
Aristotle was the student of Plato who was the student of
Socartes.
Aristotle was an accomplished individual whose work would
influence the course of history:
1)
Teacher – Most Famous Student : Alexander the Great
2)
First Biologist
3)
Cosmologist
4)
Physicists - Developed Aristotelian Mechanics
5)
Philosopher
Aristotle
Aristotle was a keen observer of nature. By looking
at nature, he attempted to deduce the workings of
nature. The weakness of Aristotle and his followers
was the complexity of the world. Unlike Galileo and
modern scientists who build controlled experiments
which are designed to restrict the number of things
which might change (temperature, pressure, volume,
etc.), Aristotle’s followers worked with systems
where hundreds of things changed.
Aristotelian Mechanics
According to Aristotle, all objects on Earth have a
natural motion (rest) and a preferred location in the
universe.
1)
2) A force (push or pull) is required to cause an
object on Earth to move or leave its natural location.
3) Applying a force to an object gives the object
“impetus.” As the object moves, it uses up impetus.
once the object is out of impetus, the object returns to
rest at its natural location.
Aristotelian Mechanics
4) According to Aristotle, the universe is composed of
five elements (different types of mater):
Earth
Water
Air
Fire
Eather
5) Outer space was filled with Eather, a fluid through
which the stars and planets traveled with no
resistance.
6) All object on Earth were composed of various
amounts of the remaining four elements.
Aristotelian Mechanics
7) The Earth is a stationary sphere at the center of the
universe.
8) The natural place of each element in the universe
was determined by observation:
Eather
Fire
Air
Water
Earth
Aristotelian Mechanics
Example: According to Aristotle, when we burn a log,
we release fire and air that was contained in the log.
The smoke and fire rise because the natural location
of fire and air is above that of earth.
Falling Body Predictions
1) Heavier objects fall toward the Earth at a greater
rate since they have more Earth element.
Falling Body Predictions
2) Each object falls to the Earth at a constant speed.
Aristitelian Mechanics
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Distance (Meters)
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Time (seconds)
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Copernicus
Platonic ideas including the Ptolemaic solar system
dominated for over 1500 years.
Nicolaus Copernicus (1473-1543) was a polish monk
who didn’t find the Ptolemaic solar system to be
satisfactory from a philosophical view due to the large
number of epicycles. Copernicus proposes a
heliocentric system of the universe with Earth being
just another planet orbiting the sun in uniform circular
motion (circle at constant speed).
Copernicus
Copernicus reserved publication of his book, On the
Revolutions of the Heavenly Spheres, till he was near
death to avoid confrontation with the church which was
dominated by Aristotelian thinkers. His work and its
consequences gave a new political meaning for the
word revolution.
Note: Most people were illiterate. The exception was
the wealthy and the clergy since they ran the
government and studied scriptures. They were well
educated in all subjects including Aristotelian
Mechanics.
Copernicus
In his book, Copernicus also included a discussion of
epicycles and other platonic ideas. Thus, his radical
ideas were almost lost. However, the heliocentric solar
system did have a powerful effect on Galileo and
Kepler. There is also some arguments supporting that
other famous people of this time became Copernicans
including Dante and Shakespeare.
Testing Models
What evidence supported the Copernican model of the
solar system?
Answer: None at this time
Galileo was a devoted Copernican and would begin
collecting data that led to the downfall of the Ptolemaic
solar system. However, when the church offered to
rule in his favor if he could prove that the Earth moved,
he had no proof.
Astrology
People of the 1500’s were strong believers in witch
craft and in astrology. The ruling class believed that
their actions were ruled by the stars. Most
academicians including Kepler made a living by
casting Royal horoscopes.
Conjunction is an astronomical event in which planets
are aligned. During the 1500’s, the minor planets were
predicted to align (looks like a very bright star).
However, the Ptolemaic tables were off a few days
and the Copernican tables were worse.
Tycho Brahe
A Danish noble who realized that the reason the
astronomical tables incorrectly predicted the date of
the conjunction was that no one had made the
necessary precise measurements of the stars.
Brahe built the best astronomical observatory and
instruments of the time. His measurements of the
locations of the stars were 5 times more precise than
previous measurements.
Tycho Brahe
Brahe was not a nice man. He especially didn’t like the
serious Johannes Kepler who he toyed with.
Brahe had a silver nose because his nose had been
cut-off during a sword fight.
On his death bed, Brahe begged Kepler to analyze his
data and to publish Brahe’s model of the solar system.
However, Brahe’s family kept the data since they
thought it might be worth money so Kepler broke in
and stole the data.
Tychonic Universe
In the Tychonic Universe, the Sun orbits the Earth, but
all other planets orbit the Sun. This model is important
because it eliminated the concept of the crystal
spheres and freed astronomers from the concept that
all motion had to be circular.
Tycho’s most important contribution was his precise
measurements since the new measurements showed
that Mar’s orbit could not be explained using a circle.
Johannes Kepler (1571-1630)
Kepler was one of the great geniuses of all time. He
developed the laws of planetary motion and almost
developed the Calculus.
Kepler had a rough life:
1. Father deserted family
2. Raised in Orphanage
3. Rough Marriages
4. Religious Persecution
5. Mother tried as a witch
6. Children were unsuccessful
Johannes Kepler
Kepler began wandering Europe on January 1, 1600.
He eventually went to see Brahe where he spent
eighteen months working for Brahe and trying to get
Brahe to show him his measurements.
Once he has Brahe’s data, he spends sixteen years
analyzing data before he finds that three statements
can describe all of the motions in the heavens
(Kepler’s Laws of Planetary Motion).
Kepler’s 1st Law
All planets travel in ellipses with the Sun at one foci.
This discovery is forced upon Kepler by the
eccentricity of the orbit of Mars which are shown in
Brahe’s data!!
x
This drawing is exaggerated for the 5 major planets
with Mars being the most eccentric at only 8% away
from a circle.
Kepler’s 2nd Law
All planets trace out equal area in equal time.
This will later be found to be a consequence of
conservation of angular momentum.
x
When a planet is far from the sun it moves slower
such that the area traced out is the same for the same
amount of time.
Kepler’s 3nd Law
The period (time required to complete a revolution –
i.e. the year) for a planet squared is proportional to the
cube of the radius of its semi-major axis.
x
a
T ka
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Kepler’s Laws
Kepler’s laws were incredibly powerful in that they
explained all know observations with just three ideas.
They also gave support to the Copernican model of
the solar system.
Kepler couldn’t explain why his laws were true!!
He simply knew that they fit the data. It will take Isaac
Newton to provide the synthesis that Aristotle had
done previously.