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History of Physics

History of Physics. A condensed version. What is Physics?. It’s a warm summer night in ancient Greece… The ancients noticed that some stars seemed to move around the sky in odd patterns, unlike most of the stars, which moved in a very regular pattern over the course of a year.

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History of Physics

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  1. History of Physics A condensed version

  2. What is Physics? It’s a warm summer night in ancient Greece… • The ancients noticed that some stars seemed to move around the sky in odd patterns, unlike most of the stars, which moved in a very regular pattern over the course of a year. • The Greeks named these stars planetes, which means “wanders”. • There were five wanders.

  3. The Ancients • Aristotle (384 – 322 BC) • Philosopher and student of Plato • Founded the Lyceum, which taught a broad range of subjects, including the study of nature • Universe made of four elements: earth, wind, fire, and water • Heavier objects fall faster than lighter objects • It was the “nature” of the sun, moon, and planets to move in circles

  4. The Ancients • Ptolemy (85 – 165 AD) • Measured and charted the positions of the planets: wrote the Almagest • Geocentric Model: Earth is the center of the universe and the sun, moon, and planets move in perfect circles around the Earth • Universe was constructed of spheres within spheres • The works of Aristotle and Ptolemy were adopted as Catholic dogma

  5. Retrograde Motion of Mars

  6. Copernicus (1473 – 1543) • Polish canon (Church lawyer) • Also studied math, astronomy, and medicine • De RevolutionibusOrbiumCoelestium (On the Revolutions of Heavenly Bodies) – Heliocentric Model • Still thought the planets must move in perfect circles, since that was the perfect shape • Better predictions and easier to use that Ptolemy’s model but considered a math trick and not reality

  7. Copernicus’ Heliocentric Model • There is no one center in the universe • The Earth’s center is not the center of the universe • The center of the universe is near the sun • The distance from the Earth to the sun is small compared to the distances to the stars • The rotation of the Earth accounts for the apparent daily rotation of the stars • The apparent annual cycle of the sun is caused by the Earth revolving around it • The apparent retrograde motion of the planets is caused by the motion of the Earth from which one observes

  8. Galileo (1564 – 1642) • Italian math professor • First person to conduct experiments • Showed that objects naturally move in straight lines at constant velocity (inertia) • Showed that all objects fall at the same rate

  9. Galileo • Did not invent the telescope but greatly improved it • First to turn the telescope to objects in the sky • mountains on the moon • four objects orbiting Jupiter • Venus had phases (like the moon) • Direct evidence the Earth is not the center of universe • Wrote a letter condemning the Ptolemic universe and claiming heliocentric was correct • Church condemned Copernicus’s model – could no longer be taught

  10. Galileo • Was given permission to compare the two world views: wrote Dialogue Concerning the Two Chief Systems of the World – Ptolemaic and Copernican. • The book took the form of two people arguing; Salviati, who argues for the Copernican and Simplicio, who argues for the Ptolemaic. • Of course, Salviati wins all the arguments. The Inquisition banned the sale of the book, ordered Galileo to appear in Rome, and sentenced him to house arrest for the remainder of his life (after agreeing to recant in exchange for his life). • 1992: Pope John Paul II admitted the Church had been wrong to condemn Galileo and his works

  11. Kepler (1571 – 1630) • German Lutheran • Learned Copernicus’ heliocentric model • Collected a very detailed catalogue of observations of the planets (mainly Mars) • Showed that the planets move in elliptical orbits, not circular • Kepler’s Laws of Planetary Motion – proved to be very accurate in predicting the motion of the planets

  12. Isaac Newton (1642 – 1727) • Already knew his three laws of motion • Since the planets move in ellipses and not straight lines, it must be because of a force • Reasoned that the same force that causes objects on Earth to fall is the same force that causes the moon to orbit the Earth • Reasoned that this force exists between all objects = universal gravitation

  13. Universal Gravitation • There is a gravitational force between all objects • Gravity is always attractive because there is only one kind of mass. • Gravity is caused by mass – more massive objects have more gravity • Gravity is weaker when objects are far apart • Planets are pulled to the Earth but are already in motion: gravity is a centripetal force in this case

  14. Henry Cavendish (1731 – 1810) • Conducted experiments to measure the universal gravitational constant G

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