Foundations-Copernican Revolution Lecture 3: Newton: Gravity and the Laws of Motion

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Foundations-Copernican RevolutionLecture 3
Newton Gravity and the Laws of Motion
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• Ancient Observatories
• Ancient Peoples around the world made places to
  mark dates from the celestial calendar and/or
  observe the sky
• Chaco Canyon, New Mexicohttp//www.exploratorium.e
  du/ancientobs/
• Chichen Itzahttp//www.exploratorium.edu/anciento
  bs/
• Angor Wat, Cambodia
• Medicine Wheel, Saskatchewan, Canada
• Stonehenge, England
• Abu Simbel, Egypt

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• Science vs. Pseudo-science
• What makes science different?
• What do we know?
• How do we know it?
• Why do we believe it?
• Is what we know supported by the evidence?
• Kuhn Paradigm
• Thomas Kuhn Structure of Scientific Revolution
• Navigating the web of a 1000 lies

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• End of the world in 2012?December 21, 2012
• See National Geographic and NASA websites on this
• Mayan Calendar long count calendar enters a new
  cycleEach cycle is 5,125.37 years
• The sun will cross in front of the center of the
  Milky WayBut this has happened many times
  beforeNo significant increase in gravitational
  interaction
• Sun will be in solar maximum more solar flares
  and stormsThis has happened may times before
• 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?

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Question 1
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.
Mars, Jupiter, and Saturn show retrograde motion
because
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Question 1
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.
Mars, Jupiter, and Saturn show retrograde motion
because
As Earth overtakes and passes the outer
planets, they seem to slow down and then reverse
direction.
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Question 2
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.
Epicycles were used in Ptolemys model to explain
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Question 2
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. .
Epicycles were used in Ptolemys model to explain
Planets were assumed to move uniformly on an
epicycle, as it moved uniformly around Earth.
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Question 3
a) craters on the Moon b) sunspots c) lunar
maria d) satellites of Jupiter e) stars of the
Milky Way
Which of Galileos initial observations was most
challenging to established geocentric beliefs?
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Question 3
a) craters on the Moon b) sunspots c) lunar
maria d) satellites of Jupiter e) stars of the
Milky Way
Which of Galileos initial observations was most
challenging to established geocentric beliefs?
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.
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Galileo (1564-1642)
Built his own telescope. Discovered four moons
orbiting Jupiter gt Earth is not center of all
things! Discovered sunspots. Deduced Sun
rotated on its axis. Discovered phases of Venus,
inconsistent with geocentric model.
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Phases of Venus
The phases of Venus are impossible to explain in
the Earth-centered model of the solar system.
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Timelines of the Big Names
Galileo
1564-1642
Copernicus
Newton
Brahe
1473-1543
1473-1543
1546-1601
1642-1727
Kepler
1571-1630
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Kepler (1571-1630)
Used Tycho Brahe's precise data on apparent
planet motions and relative distances. Deduced
three laws of planetary motion.
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Question 4

1. Earth orbits slower in January.
2. Earth orbits faster in January.
3. Earths orbital speed doesnt change.

Earth is closer to the Sun in January. From this
fact, Keplers 2nd law tells us
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Question 4

1. Earth orbits slower in January.
2. Earth orbits faster in January.
3. Earths orbital speed doesnt change.

Earth is closer to the Sun in January. From this
fact, Keplers 2nd law tells us
Keplers 2nd law means that a planet moves faster
when closer to its star.
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Kepler's First Law
The orbits of the planets are elliptical (not
circular) with the Sun at one focus of the
ellipse.
Ellipses eccentricity (flatness of ellipse)
distance between foci
major axis length
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Kepler's Second Law
A line connecting the Sun and a planet sweeps out
equal areas in equal times.
faster
slower
Translation planets move faster when closer to
the Sun.
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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 a3
or
P2 ? a3 (for circular orbits,
abradius). Translation the larger a planet's
orbit, the longer the period.
a
b
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1.3 The Laws of Planetary Motion
Keplers laws
3. Square of period of planets orbital motion is
proportional to cube of semimajor axis.
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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.
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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.
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Newton's First Law of Motion

• Last time Demo of objects at rest
• Demo nature of how objects move
• Hover disk

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1.4 Newtons Laws
Newtons laws of motion explain how objects
interact with the world and with each other.
Newtons 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.
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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 m
a
or Fnet ma
acceleration is a change in velocity or a change
in direction of velocity.
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Newton's Second Law of Motion

• Demo - Force and Acceleration
• Fan Carts