GRAVITY

Planetary Motion

• The motions of the planets in a heliocentric solar system can be encapsulated by a 3 simple rules - Kepler's Laws:
  1. The orbits of planets are elliptical in shape.
  2. A line from the sun to a planet sweeps out equal areas in equal times.
  3. The square of the period of a planet's orbit is proportional to the cube of the size of that orbit.
• Kepler's Laws were a great simplification over epicycles.
• Kepler's Laws were found to emerge naturally from

• Newton's Universal Law of Gravitation:
  • Everything happens... as if the force between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

  

• Newton's Law of Gravity predicts
  • Arbitrary orbits:
    • Planets well into future
    • Moons of other planets
    • Comets (Halley; Hyakutake)
    • Satellites we launch into orbit
  • Motions on Earth:
    • The falling apple
    • Baseballs
    • Bombs
    • Etc...