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Unformatted text preview: 1 “Gravity and Forces” 1) Review Greek Astronomy Ptolemy & Copernicus Kepler’s Laws 2) Newton’s Laws 3) Gravity Examples : Weightlessness Tides & Tidal forces 4) Summary Today’s Lecture: purpose & goals AST 1002 Planets, Stars and Galaxies Review Prograde Motion Retrograde Motion Greek Astronomy • Eratosthenes of Alexandria 276-194 BC •Determined the circumference of the Earth . Could you prove that the Earth is a sphere? (and how would you do it?) • Claudius Ptolemius , Alexandria 140 AD • Geocentric Model of the Universe-- Earth c entered-- based on Aristotilean ideas of truth, beauty, and symmetry-- Epicycles & Deferents (cycles upon cycles upon cycles) Golden Age of Astronomy • Nicholas Copernicus Polish astronomer 1473-1543 • Heliocentric Model of the Universe-- Sun c entered-- still circular motion-- Explained retrograde motion much more simply • Tycho Brahe Danish nobleman; eccentric 1546-1601-- greatest pre-telescope astronomer-- three decades of observations of planetary motion • Johannes Kepler “the wandering mathematician” 1571-1630-- used Brahe’s data (primarily of Mars ) to build 3 Laws of Orbital Motion Keplers First Law: Planets move in orbits which are ellipses with the Sun at one of the focal points Kepler’s Second Law: Planets sweep out equal areas in equal times -- fastest when nearest to the Sun; slowest when farthest from the Sun!! Kepler’s Third Law: The square of a planet’s orbital period is proportional to the cube of the orbit’s semi-major axis ( P years ) 2 = ( A AU ) 3 or P 2 = (4 π 2 / G . M ) . A 3 Review 2 Kepler’s Laws of Orbital Motion Remember that Kep l er s L aw s are emp i r i c a l – they s how HOW obje c t s move , but not why . The rea s on s WHY obje c t s move i s GRAVITY , wh i c h i s the top i c o f today ’ s l e c ture . 2 But WHY??? Kepler’s Laws do an excellent job of describing the motion of the planets but they don’t tell us why the planets do this Science tries to learn how and why Two great scientists helped answer these questions Galileo Galilei (1564-1642) Sir Isaac Newton (1642-1723) Newton’s Laws of Motion satellite: Constant velocity is along a straight line. “Rest” is zero velocity. also called inertia . Newton’s First Law The first part seems obvious you stay in your seat unless you make yourself get up or someone/something pushes you. you do not mysteriously go flying out of your seat. The second part seems less obvious you are used to an object sliding across the floor and slowing down . but it really does want to keep moving (the sliding is a frictional force ) Now we need some definitions … An object at rest will stay at rest, an object in motion will remain in motion …unless an unbalanced force acts upon it....
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