Topic 8: Planetary motion and gravity

• Aphelion: the point in an orbit when a planet is furthest from the sun
• Perihelion: the point in an orbit when a planet is closest to the sun
• Apogee: the point in a orbit when an object is furthest from the earth
• Perigee: the point in an orbit when an object is closest to the earth
• Kepler's first law: Every planet orbits the Sun in an elliptical orbit with the Sun at one of the foci
• Kepler's 2nd law: an imaginary line between a planet and the sun sweeps out equal area in equal time
• Kepler's 3rd law: The square of the orbital period of a planet is proportional to the cube of its mean distance from the sun
• Newton's law of gravitation: Every body in the universe attracts every other body with a force that is directly proportional to the product of their masses and inversely proportional to the square of their distance apart
• Consequences of Newton's law:

  • Double mass, double force
  • Double distance apart, quarter force
  • Halve mass, halve force
  • Halve distance apart, quadruple force