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UW-Madison PHYSICS 107 - LECTURE NOTES

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1Physics 107, Fall 2006 1Physics 107Ideas of Modern Physics• Main emphasis is Modern Physics: Post-1900 Physics• Why 1900?– Two radical developments:Relativity & Quantum Mechanics• Both changed the way we think as much asdid Galileo and Newton.(www.hep.wisc.edu/~herndon/107-0609)Physics 107, Fall 2006 2Goals of the course• Learn a process for critical thinking, and apply it to evaluate physical theories• Use these techniques to understand therevolutionary ideas that embody modern physics.• Implement the ideas in some basic problems.• Understand where physics is today,and where it is going.Physics 107, Fall 2006 3What will we cover?• Scientific observation and reasoning.• Motion and energy.• Relativity.• Quantum Mechanics.•Gravity.• Particle theory and cosmology.Physics 107, Fall 2006 4From the microscopically smallSingle atomsand quantum wavesTo the incredibly largeEntire galaxies and theuniverseModern Physics:Physics 107, Fall 2006 5How do we do this?•Lectures• Demonstrations• In-class interactive questions• Homework• Discussion sectionsHW 1: Chap 3 Conceptual 6, 28, 32Chap 3 Problems 6, 10, 16Physics 107, Fall 2006 6What do you need to do?• Read the textbook Physics Concepts and Connections• Come to the lectures 9:55 MWF in 2241 Chamberlin Hall• Participate in discussion section One per week, starting Sept 11th•Do the homework Assigned most Wednesdays, due the following Wednesday• Write the essay On an (approved) physics topic of your choice, due Dec 8• Take the exams Three in-class hour exams, one cumulative final exam2Physics 107, Fall 2006 7What do you get?• An understanding of the physical universe.•A grade– 15% HW and Discussion Quizzes– 15% essay– 20% each for 2 of 3 hour exams (lowest dropped)– 30% from cumulative final examPhysics 107, Fall 2006 8Where’s the math?• Math is a toolthat can often help to clarify physics.• In this coursewe use algebra and basic geometry.•We will do calculations, but also focus onwritten explanation and reasoning.Physics 107, Fall 2006 9Observation and Science• Look around - what yousee is the universe.• What can you say abouthow it works?Physics 107, Fall 2006 10Aristotle’s ideas about motion• Terrestrial objects move in straight lines.Earth moves downward, Water downward,Air rises up, Fire rises above air.• Celestial bodies are perfect.They move only in exact circles.• Where did Aristotle concentrate his work?– Celestial bodies, most interesting problem of the dayPhysics 107, Fall 2006 11Motion of the celestial bodiesApparent motion of stars:Rotation about a pointevery 24 hours.Moon, sun, and planetswere known to move withrespect to the stars.Physics 107, Fall 2006 12Motion of the stars over 6 hrs3Physics 107, Fall 2006 13Daily motion of sun & planets over 1 yearMovie by R. Pogge,Ohio StatePhysics 107, Fall 2006 14Aristotle’s crystal spheresEarth/Water/Air/FireMoon (28 days)Mercury (1 yr)Venus (1 yr)Sun (1 yr)Mars (2 years)Jupiter (12 years)Saturn (30 years)Firmament (1000 yrs)Prime mover (24 hrs)Cristal sphere (49000 yrs)Already Complex!Physics 107, Fall 2006 15You figure it out!Assuming that the planets and stars are movingaround the earth you would expect:A. The planets to move faster than the starssince they are closer.B. The stars to move faster than the planets.C. We wouldn’t know what to expect.I would say it would be helpful to have moreinformation!Physics 107, Fall 2006 16Detailed Observations ofplanetary motion (Ptolemy)Observational notes from Ptolemy’s Almagest85-165An instrument similarto Ptolemy’sPhysics 107, Fall 2006 17Retrograde planetary motionContinued observation revealedthat the problem was even morecomplex than first believed!Retrograde motion of Mars.Apparent motion not always in aperfect circle.Mars appears brighter during theretrograde motion.Physics 107, Fall 2006 18Epicycles, deferents, and equants:the legacy of PtolemyEpicycle reproduced planetary retrograde motion4Physics 107, Fall 2006 19Ptolemy’s universe• In ‘final’ form– 40 epicycles anddeferents–Equants andeccentrics for sun,moon, and planets.– Provided detailedplanetary positionsfor 1500 years– Very complex!– However good forwhat was needed, navigation.Physics 107, Fall 2006 20More detailed observations, +some philosophy (Copernicus)• Ptolemy’s system worked, but seemed alittle unwieldy, contrived.• Required precise coordination of planetarypaths to reproduce observations.• Imperfect circular motionagainst Aristotle.• Copernicus revivedheliocentric (sun-centered) universe1473-1543Physics 107, Fall 2006 21The heliocentric universe• Sun-centered• Planets orbitingaround sun.• Theory didn’tperfectly predictplanetary motion.Only discovered later.• But the (imperfect)theory is attractive inseveral ways.Physics 107, Fall 2006 22Advantage: “Natural”explanation of Retrograde motionRetrograde motion observed asplanets pass each other.Physics 107, Fall 2006 23Comparing Ptolemy and CopernicusPtolemy’s Earth-centeredCopernicus sun-centeredWhich is the better theory?Physics 107, Fall 2006 24How can we tell if it is ‘correct’?But a rotating and revolving Earth seemed absurd!Both motions require incredibly large speeds: Speed of rotation ~ 1280 km/hour Orbital Speed: 107,000 km/hr = 30 km/sec!No observational evidence of orbital motion: Relative positions of stars did not shift with Earth’s motion (parallax) Stars weren't brighter when Earth is closer (opposition).No observational evidence of rotation: Daily motions are as easily explained by a fixed earth. The motions do not require a rotating earth.Both explained contemporaneous observations.5Physics 107, Fall 2006 25Advantage:A ‘good’ theory makes predictionsEarthhalf-illuminatedVenus9.549.17Saturn5.205.22Jupiter1.521.52Mars1.001.00Earth0.7230.719Venus0.3870.376MercuryActualCopernicusPlanetBut, at the time, these predictions could not be tested!Physics 107, Fall 2006 2620 years of detailed observations(Tycho Brahe & Johannes Kepler)• Brahe’s exacting observationsdemanded some dramaticrevisions in planetary motions.Both Ptolemy’s andCopernicus’ theorieswere hard-pressed atthis detailed level.1546-1601Physics 107, Fall 2006 27Kepler’s elliptical orbits• Contribution of Kepler:– first consideration of non-circular orbits in over1000 yrs of thinking.– No more epicycles required!1571-1630CircularorbitElliptical orbitDetailed observations


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