Historically very interesting, heliocentric vs . geocentric universe

1. Chapter 13: The Law of Gravity Reading assignment: Chapter 13.1 to 13.5 Homework (due Wednesday, Nov. 14): OQ1, OQ2, OQ5, OQ8, 2, 3, 6, 10, 13, 14, 18, 28, 33 • Historically very interesting, heliocentric vs. geocentric universe • The main cast: Copernicus, Brahe, Galileo, Kepler, Newton • Satellite motion Copernicus 1473 – 1543 Brahe 1546 – 1601 Galileo 1564 – 1642 Kepler 1571 – 1630 Newton 1643 – 161727

2. Geocentric vs. heliocentric model of earth • Earth is round! Slowly accepted, starting in ancient Greece, Magellan + Elcano circumnavigation (1519 – 1522). • Ptolemy (100 –170 A.D.) geocentric model: Sun revolves around earth (Wrong!) • From astronomical observations: • Copernicus (1473-1543) heliocentric model: Earth & planets revolve around sun • Brahe (1546 - 1601) Accurate observation of planetary motion • Galileo (1564 - 1642) (1610) supports the heliocentric model • Kepler(1571-1630), 1609: Laws I, II of planetary motion, • Kepler 1619: Law III of planetary motion About falling objects • Aristotle (384-322 B.C.) Heavier objects fall faster than light objects (Wrong!) • Galileo (1564 - 1642) Neglecting air resistance, all objects fall at same acceleration

3. G… Gravitational constant G = 6.673·10-11 N·m2/kg2 m1, m2…masses of particles 1 and 2 r… distance separating these particles … unit vector in r direction Experimentally confirmed. Measuring the gravitational constant – Cavendish apparatus (1789) Newton’s Law of Universal Gravitation Every particle in the Universe attracts every other particle with a force of:

4. q Black board example 13.1 What is the attractive force you (m1 = 100 kg) experience from the two people (m2 = m3 = 70 kg) sitting in front of you. Assume a distance r = 0.5 m and an angle q = 30° for both?

5. Free-Fall Acceleration and the Gravitational Force Gravitational force: Thus: g is not constant as we move up from the surface of the earth! G is a universal constant (does not change at all).

6. Black board example 13.2 Variation of g with altitude The ISS photographed from shuttle Discovery in 2006. From http://www.daviddarling.info/encyclopedia/I/ISS.html • What is the value of g in the ISS space station that is at an altitude of 400 km. Assume ME = 5.960·1024 kg and RE = 6.370·106 m. • Why does it feel like g = 0?

7. Gravitational potential energy (similar equation for charge-charge interaction) • Notice the – sign • U = 0 at infinity • U will get smaller (more negative) as r gets smaller. • “Falling down” means loosing gravit. potential energy. • Use only when far away from earth; otherwise use approximation U = mgh.

8. Black board example 13.3 First Rocket Launch from Cape Canaveral (NASA); July 1950 • What is the escape speed of a particle on earth (ignore air resistance)? • A) ~10,000 m/s B) ~11,000 m/s C) ~20,000 m/s D) ~22,000 m/s

9. Kepler’s laws about planetary motion These laws hold true for any object in orbit around a central mass • Kepler’s first two laws (1609): • Planets move in elliptical paths around the sun. The sun is in one of the focal points (foci) of the ellipse. • The radius vector drawn from the sun to a planet sweeps out equal areas in equal time intervals (Law of equal areas). Area S-A-B equals area S-D-C

10. Kepler’s laws about planetary motion Kepler’s third law (1619): III. The square of the orbital period, T, of any planet is proportional to the cube of the semimajor axis of the elliptical orbit, a. G… Gravitational constant G = 6.673·10-11 N·m2/kg2 MS … central mass, (i.e. mass of sun for planet motion) Thus, for any two planets:

11. All nine eight planets of the solar system

12. Inner solar system: Mercury Venus Earth Mars Outer solar system: Jupiter Saturn Uranus Neptune (Pluto) My Very Educated Mother Just Served Us Nine (Plums)