Lecture #6: Gravity gets you down (Sep 2)
I. Teaser: demonstration that all objects fall at same rate
II. Orbits and ellipses
-- how to make an ellipse (circle is special case)
-- definition of semi-major axis, eccentricity, focus
-- as ellipses, orbits are "closed"-- they return to same point!
-- if speed exceeds "escape speed", path is no longer closed-- hyperbola
III. Kepler's laws (all true in limit that Sun mass is huge)
1) orbits are ellipses with the Sun at one focus
2) orbit sweeps out equal area in equal time
3) P^2/a^3 = constant
-- note: what is conspicuously absent from this expression?
- planet's mass! Recall teaser above... could there be a connection??