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The History and Philosophyof Astronomy (Lecture 13: Newton I)Instructor: Volker BrommTA: Amanda BauerThe University of Texas at AustinAstronomy 350L (Spring 2005)Isaac Newton: Founding Father of Physics • 1642 (Woolsthorpe) – 1727 (London)• Principia MathematicaPhilosophiae Naturalis(“Mathematical Principles ofNatural Philosophy”, 1687)- universal gravity (inverse-square law)- three laws of motion• invented calculus(differentiation and integration)Newton: Timeline and Context • building upon Galileo, Kepler, and Descartes• completes Copernican Revolution!DescartesNewton: Geography of his Life N D1642: Birth in Woolsthorpe• born in rural Lincolnshire• father died before his birth (‘posthumous child’)1642 –49: The English Civil War • bitter struggle between King (Charles I Stuart)and Parliament (“Cavaliers” vs “Roundheads”)• King desires to rule without Parliament1649: Execution of the King • King Charles I (Stuart) beheaded1642 –49: The English Civil War • Victory for Parliament• Republic (“Commonwealth”)• Oliver Cromwell (1599-1658)- Lord Protector• Anarchy after his death• Army recalls son of former (executed) kingfrom exile1660: The Restoration • Return of the Stuarts: Charles II (son of behead king)London Coffee-House Culture • New venue for meetings of intellectualsThe Royal Society of London • founded 1660: institution to foster exchange ofscientific knowledgeGreshamCollegePhilosophical Transactions • published by Royal Society• first scientific journal• a public registry of newscientific ideas• professionalization of science1661: Newton enters Cambridge University • one of oldest universities in the world1661: Newton admitted to Trinity College • admitted as “subsizar” (has to perform menial duties for older or richer students)Student in Cambridge (1661-65) • Study Descartes’mechanical philosophy!• Principia Philosophiae(1644)• No vacuum, no atoms!• Force by direct contact(pressure and tension)Student in Cambridge (1661-65) • Study all the mathematicsthat there is to know!• John Wallis ArithmeticaInfinitorum(1656): - predecessor of integralcalculus- introduces symbol forinfinity ( )John Wallis, 1616-170381665: The Great Plague ~100,000 dead in London1666: The Great Fire of LondonChristopher Wren: England’s Greatest Architect • 1632 – 1723• Rebuilt London after Great Fire of 1666• > 50 new churches• St Paul’s Cathedral• Savilian professor ofastronomy at OxfordWren: Rebuilding London • St Paul’s CathedralNewton during Plague Year: AnnusMirabilis • Return to Woolsthorpe• 3 Great Discoveries:- Calculus- Nature of Light- Universal Gravity “The Miraculous Year”(1665-66)AnnusMirabilis I: Calculus DifferentiationIntegrationyx• independently discovered by Leibniz in Germany(giving rise to ugly priority dispute later on…)AnnusMirabilis II: Optics • white light is composed of different colors!AnnusMirabilis III: Universal Gravity • Newton asks: What if the same force (gravity) causesfall of apple and keeps Moon in orbit around Earth???AnnusMirabilis III: Universal Gravity • Moon is constantly falling toward Earth (as is apple)! F~v2/rCentripetal forcevrExperiencing the Centripetal ForceAnnusMirabilis III: Universal Gravity • Earth’s gravitational pull is ~ 1/3600 weakerat location of Moon compared to surface (apple)!AnnusMirabilis III: Universal Gravity • Kepler 3 + Galilean inertia = inverse-square law! F~v2/rCentripetal forceP2=r3Kepler’s 3rdLaw +F ~ 1/r2Inverse-square law }=AnnusMirabilis III: Universal Gravity • Intuitive nature of inverse-square law!(compare to dilution of light over growing surface)AnnusMirabilis III: Universal Gravity • Inverse-square law for force of gravityBig remaining problem: • Does it also work for elliptical orbits???Newton’s Return to Cambridge• 1669: Lucasian Professor for MathematicsIsaac Barrow: 1stLucasian ProfessorStephen Hawking: 17thLucasian ProfessorNewton’s Return to Cambridge • 1671: Design for new (reflecting) telescope • earns him membership (as fellow) in Royal SocietyEn Route to the Principia • Newton was very reluctant to publish! • Edmond Halley (1656-1742)• member of Royal Society• Halley’s Comet• first astronomer to observeSouthern Sky (from St Helena)• Convinced Newton to publish PrincipiaEn Route to the Principia • Remove difficulty 1: - inverse-square law and elliptical orbits! • Result: Inverse square-law explains all of Kepler’s laws!En Route to the Principia • Remove difficulty 2: - gravitational force near surface of Earth!Newton’s Principia (1687) • Philosophiae NaturalisPrincipia Mathematica(Mathematical Principles of Natural Philosophy)• Challenges Descartes’Principia Philosophiae (1644)- Descartes: qualitative- Newton: quantitative, predictive• The foundational text formodern physics and astronomy!Newton (part 1)• Isaac Newton: - founder of modern physics and astronomy- led reclusive anti-social life in Cambridge- made Lucasian Professorship (Cambridge) famous• Invented Calculus- differential and integral calculus- independently discovered by Leibniz (priority dispute)• Principia - the foundational text for modern physics and astronomy- laws of motion- universal gravity (inverse-square


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