Transcript Aristotle (384-322 B.C)

Aristotle
(384-322 B.C)
• Greek philosopher, scientist,
and educator.
• Systematized existing
knowledge:
• Made observations
• Collected specimens
• Gathered together,
summarized, and classified
existing knowledge of the
physical world
Aristotle
(384-322 B.C)
• Believed the Earth was a
sphere because:
– The Earth’s shadow on
the Moon during a
lunar eclipse is always
round.
Aristotle
Aristotle
(384-322 B.C)
(384-322 B.C)
• Believed the Earth
as sphere because:
– Stars near the
celestial pole are
higher in the sky the
farther north you
travel.
Aristotle
(384-322 B.C)
• Believed the Earth was a sphere because The fact
that objects fall to Earth towards its center means
that if it were constructed of small bits of matter
originally, these parts would naturally settle into a
spherical shape by compression
Ships Sailing Over the Horizon
Aristotle on Motions
Violent Motion
Natural Motion
Natural Motion
• Is caused by
objects “striving”
to get to their
proper place.
• The “proper
place” is
determined by
how much of each
of the fours
elements (Earth,
water, air, fire) the
object contains.
Smoke (made of Air)
strives to get to the Air.
A feather (part Earth, part
Air) strives to get to the
Earth, but is slowed by
also striving to get to the
Air.
A rock (made of
Earth) strives to get
to the Earth.
Violent Motion
• Results from
something external
pulling or pushing
on an object in
opposition to its
natural motion
• Requires a
constant push or
pull to sustain the
motion
Once in flight,
air filling in
behind and
pushing on the
arrow keeps it
going
Lifting the rock
opposes the
rock’s striving
to the Earth.
Some Aristotelian Conclusions
• A heavier object
(having more earth)
will fall to the
ground faster than a
light object.
• If you stop pushing
an object, it will stop
moving.
Aristotle’s Universe
• The Earth is stationary and
at the center.
• The heavens are 55
concentric crystalline
sphere, each carrying
celestial objects and
rotating.
• Celestial bodies are perfect
spheres made of a perfect
and unchanging substance
(quintessence).
• A “Prime Mover” keeps the
heavens in motion.
Eratosthenes
(276-195 BC)
At noon on the Summer Solstice, sunlight struck
the bottom of a vertical well in Syene, Egypt.
At that moment, the
shadow cast by a vertical
obelisk in Alexandria
(5,000 stadia distant)
showed that the Sun was
7.2 degrees away from
the vertical.
Therefore, the Earth’s circumference is
5000 x (360/7.2) = 250,000 stadia
How Accurate Was Eratosthenes?
• Historians disagree on
how big a Greek
“stadium” was.
• Depending on what the
correct conversion to the
modern units is,
Eratosthenes’s estimate
was between 1 and 20
percent larger than the
modern value.
Why Does It Matter?
The panel of experts appointed by
King Ferdinand to review
Columbus’s proposal to sail to
India was aware of Eratosthenes’s
work. Because of this, they ruled
that the distance was too great and
that he therefore should not go.
Columbus, using either error-laden
estimates of others, or using an
incorrect conversion factor, believed
that the distance was only 1/3 or ½
of the true distance.
Queen Isabella overruled the
experts and sided with Columbus.
Beginnings of modern, empirical science:
Ibn al-Haythem (965-1040 CE)
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Born in Basra, Iraq
Educated in Basra and Baghdad
Worked in Egypt and Spain
Developed the modern theory of optics by taking
careful measurements in repeatable
experiments
• Considered various theories of light and
designed and carried out experiments to
determine which theory might be more accurate
Ibn al-Haythem
• Studied the propagation of light and the nature
of colors
• Studied optic illusions and reflections
• Studied the refraction (bending) of light rays
when pass from one transparent medium to
another (air, water) and studied the angle of
incidence and refraction quantitatively
• Dispersion of light into colors
• Studied the way the light travels through the
atmosphere and estimated the thickness of the
atmosphere
Beginnings of modern, empirical
science:
Roger Bacon (1214-1294 CE)
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Born in England, worked in Paris and Oxford
Became a Franciscan friar
Vigorously encouraged experimental science
Asserted that the study of the natural world by
observation and exact measurement was the
surest foundation for truth.
• Having read al-Haythem’s works, he performed
optical experiments with lenses and mirrors,
described spectacles
Roger Bacon
“The strongest arguments prove nothing so
long as the conclusions are not verified by
experience. Experimental science is the
queen of sciences and the goal of all
speculation.”
“For the things of this world cannot be made
known without a knowledge of mathematics”
All science requires mathematics.
Galileo Galilei
(1564-1642)
• Defined new concepts in
addition to discovering
objects
• Simplified investigations
through:
– Reducing scope (focusing on
one issue at a time)
– Idealization (thinking away
complications)
• Designed experiments to
test hypotheses
• Described the physical
world with mathematics
Galileo’s approach sets the
stage for all of modern
science.
Galileo on authority, measurement, and
math
In questions of science the authority of a thousand is not
worth the humble reasoning of a single individual.
Count what is countable, measure what is measurable,
and what is not measurable, make measurable.
Philosophy is written in this grand book, the universe,
which stands continually open to our gaze. But the book
cannot be understood unless one first learns to
comprehend the language and read the letters in which it is
composed. It is written in the language of mathematics,
and its characters are triangles, circles, and other
geometric figures without which it is humanly impossible to
understand a single word of it; without these, one wanders
about in a dark labyrinth.
Galileo on Falling Bodies
• Experiment: drop objects
of various weights from
the Leaning Tower of
Pisa
• Conclusion: Except for
small effects of air
resistance, objects of
various weights fall
together and reach the
ground at the same time.
Aristotle observed that there
was always a resistive medium
in the world around him.
Galileo imagined what would
happen if there was no
resistance.
Galileo on Motion
• Experiment: Observe motions of
various objects going down inclined
planes.
• Conclude: In the absence of friction,
a rolling ball rolling along a
horizontal surface:
– will neither speed up nor slow down
– will never come to rest.
Aristotle: motion requires a
push or a pull to keep moving.
Galileo: with no interference, a
moving object will kepp moving
in a straight line forwever.
Inertia
• Galileo defined a new word
Inertia: the property of an object that
tends to keep it moving straight
ahead or that resists changes in it
motion
Galileo’s Universe
• Seeing that Jupiter had
moons orbiting meant
that celestial bodies
(such as our Moon)
could orbit objects other
than our Sun.
• Extending the idea of inertia
to circular motion meant that
the Earth could circle the
Sun indefinitely.
• So Galileo adopted the Copernican model of the
universe
“Mathematics is the language with which God has
written the universe.”
“Philosophy is written in this grand book - I mean
the Universe - which stands continually open to
our gaze, but it cannot be understood unless one
first learns to comprehend the language and
interpret the characters in which it is written. It is
written in the language of mathematics, and its
characters are triangles, circles and other
geometrical figures, without which it is humanly
impossible to understand a single word of it.”
Galileo