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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 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 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 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 cycle Each cycle is 5,125.37 years 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 Sun will be in solar maximum – more solar flares and storms This 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? 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. . 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 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 • Fan Carts