2. Gravity How come the astronauts float around when they are inside the space shuttle? A) The shuttle’s rockets compensate for gravity B) The shuttle is so far from earth that gravity is very, very weak. C) Earth’s atmosphere shields the shuttle from gravity D) The Moon’s gravity compensates for the Earth’s E) NASA didn’t pay the gravity bill F) There is nothing holding the shuttle up Orbits

3.  What happens when something holds you up.  Gravity pulls you down  Floor pushes you up  Forces balance - you don’t go anywhere.  You know which way is up because that is the way the floor is pushing you Gravity and Orbits - Not Free Fall Help! Cut the cable The elevator starts falling The floor no longer holds you up You fall too You float around – no gravity Because you and elevator are falling at the same rate, feels like weightlessness Free fall can be up, down, or sideways

4. “The knack [to flying] lies in learning how to throw yourself at the ground and miss” - The Hitchhiker’s Guide to the Galaxy trilogy, in Life, the Universe, and Everything Gravity and Orbits - Circular Orbits

5. Gravity and Orbits How come the astronauts float around when they are inside the space shuttle? A) The shuttle’s rockets compensate for gravity B) The shuttle is so far from earth that gravity is very, very weak. C) The Earth’s atmosphere shields the shuttle from gravity D) The Moon’s gravity compensates for the Earth’s E) NASA didn’t pay the gravity bill F) There is nothing holding the shuttle up

6. Non-circular orbits If velocity is more or less than this, you get an ellipse If velocity is big enough, it leaves on a parabola – escape velocity If velocity is bigger, it leaves on a hyperbola

7. Two Body Orbits Center of Mass The Center of Mass is where the two masses would balance The objects go around each other How come the Sun doesn’t go around the Earth? It does! (M + m)P 2 = a 3

8. Announcements 5/28 DateReadQuiz TodaySec. 4.1 – 4.4,yes Gravity Lecture MondaySec. 5.1 yes TuesdaySec. 5.2 – 5.3yes Test next Thursday! Information sheet online soon?

9. Kepler vs. Newton (M + m)P 2 = a 3 P 2 = a 3 Sun + Earth M + m = 1 +.000003  1 Applies to planets around the Sun Always applies Masses in Solar Masses

10. Gravity vs. Pressure = sphere Gravity wants to squish everything to a point Pressure wants it to spread out They balance to make a sphere If an object rotates, it makes a flattened (oblate) sphere Small objects can be any shape

11. 26,000 year cycle Precession Gravity pulls unevenly on the flattened Earth Earth is like a top – its tilt changes over time

12. Gravity vs. Rotation = Disk  Dust or gas or chunks - disk forms  Rotation keeps it from falling in  Gravity keeps it from escaping Rings around Planets Spiral Galaxies Accretion disks Protoplanetary disks

13. Tidal Forces

14. Tidal Distortion (greatly exaggerated)

15. Big Tides Small Tides Spring (big) and Neap (small) Tides

16. Tides Generally, high tide occurs twice every day. Why? A) There are two causes of tides, the Sun and the Moon B) The Moon causes two places on the Earth’s oceans to bulge and have high tide C) The second high tide is sort of an echo of the first tide D) The Moon goes around the Earth twice every day

17.  Rotation causes bulge to be in wrong place  Bulge has gravitational pull on Moon  Earth rotation slows  Moon slung to higher, slower orbit Bulges and the Earth/Moon system Earth Moon without rotation Earth  Earth and Moon bulge want to align  Strong effect on the Moon with rotation

18. Tidal locking Why can we only see one side of the Moon? A) Because the Sun only shines on one side of the Moon B) Because the Moon doesn’t rotate, so we never see the back side of the Moon C) The Moon rotates at the same rate as it goes around the Earth D) It is like a rainbow; it doesn’t have another side Moon Earth

19. planet moon Tidal Heating  Changing shape as distance changes  Flexing generates heat

20. Planet Moon Roche Limit

21. Roche Limit - Rings

22. Where is Titan [a very large moon of Saturn] compared to Saturn’s rings? A) It is closer than the rings B) It is farther away than the rings C) It is within one of the rings D) It is between rings - there are rings both inside and outside it. Roche Limit - Rings