Transcript Today`s Powerpoint

Foundations-Copernican Revolution
Lecture 3:
Newton: Gravity and the Laws of Motion
Ancient Observatories
Ancient Peoples around the world made places to mark dates from the
celestial calendar and/or observe the sky
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Chaco Canyon, New
Mexicohttp://www.exploratorium.edu/ancientobs/
Chichen Itza
http://www.exploratorium.edu/ancientobs/
Angor Wat, Cambodia
Medicine Wheel, Saskatchewan, Canada
Stonehenge, England
Abu Simbel, Egypt
Science vs. Pseudo-science
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What makes science different?
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What do we know?
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How do we know it?
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Why do we believe it?
Is what we know supported by the evidence?
Kuhn Paradigm
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Thomas Kuhn: Structure of Scientific Revolution
Navigating the web of a 1000 lies
End of the world in 2012?
December 21, 2012
See National Geographic and NASA websites on this
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Mayan Calendar – long count calendar enters a new cycle
Each cycle is 5,125.37 years
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The sun will cross in front of the center of the Milky Way
But this has happened many times before
No significant increase in gravitational interaction
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Sun will be in solar maximum – more solar flares and storms
This has happened may times before
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A tenth planet known to the ancients that would hit us by end of
the year would be visible to astronomers for past decade
Why the hype?
Question 1
Mars, Jupiter, and
Saturn show retrograde
motion because
a) planets move on epicycles.
b) planets orbit the Sun in the same
direction.
c) Earth moves faster in its orbit.
d) they are closer than Uranus.
e) they rotate quickly on their axes.
Question 1
Mars, Jupiter, and
Saturn show retrograde
motion because
a) planets move on epicycles.
b) planets orbit the Sun in the same
direction.
c) Earth moves faster in its orbit.
d) they are closer than Uranus.
e) they rotate quickly on their axes.
As Earth overtakes and “passes”
the outer planets, they seem to
slow down and then reverse
direction.
Question 2
Epicycles were used
in Ptolemy’s model
to explain
a) why planets moved in the sky.
b) why Earth was at the center.
c) why retrograde motion occurred.
d) why Earth wobbled on its axis.
e) why inner planets were always seen
near the Sun.
Question 2
Epicycles were used
in Ptolemy’s model
to explain
a) why planets moved in the sky.
b) why Earth was at the center.
c) why retrograde motion occurred.
d) why Earth wobbled on its axis.
e) why inner planets were always seen
near the Sun.
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Planets were assumed to move
uniformly on an epicycle, as it
moved uniformly around Earth.
Question 3
Which of Galileo’s initial
observations was most
challenging to established
geocentric beliefs?
a) craters on the Moon
b) sunspots
c) lunar maria
d) satellites of Jupiter
e) stars of the Milky Way
Question 3
Which of Galileo’s initial
observations was most
challenging to established
geocentric beliefs?
a) craters on the Moon
b) sunspots
c) lunar maria
d) satellites of Jupiter
e) stars of the Milky Way
Seeing four moons clearly move
around Jupiter disproved that
everything orbited Earth
and
showed Earth could orbit the Sun and
not lose its moon, too.
Galileo (1564-1642)
Built his own telescope.
Discovered four moons orbiting Jupiter =>
Earth is not center of all things!
Discovered sunspots. Deduced Sun
rotated on its axis.
Discovered phases of Venus, inconsistent
with geocentric model.
Phases of Venus
The phases of
Venus are
impossible to
explain in the
Earth-centered
model of the solar
system.
Timelines of the Big Names
Galileo
Copernicus
1473-1543
1564-1642
Brahe
1546-1601
Kepler
1571-1630
Newton
1642-1727
Kepler (1571-1630)
Used Tycho Brahe's precise data on
apparent planet motions and relative
distances.
Deduced three laws of planetary
motion.
Question 4
Earth is closer to the Sun in
January. From this fact,
Kepler’s 2nd law tells us
a) Earth orbits slower in
January.
b) Earth orbits faster in January.
c) Earth’s orbital speed doesn’t
change.
Question 4
Earth is closer to the Sun in
January. From this fact,
Kepler’s 2nd law tells us
a) Earth orbits slower in
January.
b) Earth orbits faster in January.
c) Earth’s orbital speed doesn’t
change.
Kepler’s 2nd law means that a
planet moves faster when
closer to its star.
Faster
Slower
Kepler's First Law
The orbits of the planets are elliptical (not circular)
with the Sun at one focus of the ellipse.
Ellipses
distance between foci
eccentricity =
major axis length
(flatness of ellipse)
Kepler's Second Law
A line connecting the Sun and a planet sweeps out equal areas
in equal times.
slower
Translation: planets move faster
when closer to the Sun.
faster
Kepler's Third Law
The square of a planet's orbital period is proportional to the
cube of its semi-major axis.
P2
is proportional to
or
P2  a3
(for circular orbits, a=b=radius).
Translation: the larger a planet's orbit,
the longer the period.
a3
a
b
1.3 The Laws of Planetary Motion
Kepler’s laws:
3. Square of period of planet’s orbital motion is
proportional to cube of semimajor axis.
At this time, actual distances of planets from Sun were
unknown, but were later measured. One technique is "parallax"
"Earth-baseline parallax" uses
telescopes on either side of Earth to
measure planet distances.
Newton (1642-1727)
Kepler's laws were basically playing with
mathematical shapes and equations and seeing
what worked.
Newton's work based on experiments of how
objects interact.
His three laws of motion and law of gravity
described how all objects interact with each other.
Newton's First Law of Motion
Last time – Demo of objects at rest
Demo – nature of how objects move
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Hover disk
1.4 Newton’s Laws
Newton’s laws of motion explain how objects interact
with the world and with each other.
Newton’s first law:
An object at rest will remain at rest, and an object will
move in a straight line at constant speed if and only if
the sum of forces that act on it are balanced.
Newton's Second Law of Motion
When a force, F, acts on an object with a mass, m, it produces an
acceleration, a, equal to the force divided by the mass.
Fnet
a=
m
or Fnet = ma
acceleration is a change in velocity or a change in
direction of velocity.
Newton's Second Law of Motion
Demo - Force and Acceleration
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