1. ASTR 1200 Announcements Website http://casa.colorado.edu/~wcash/APS1200/APS1200.html.. First Problem Set Assigned. Due next Tuesday in class. Observatory Sessions all now at 8:30pm Lecture Notes going up on the website Schedule has been updated. Exam dates set. Text reading still needed..

2. Orbits Falls Freely Hits Surface Misses Surface From Sideways Motion

3. Physics of Orbits Earth sideways velocity central acceleration If you balance things just right, you have an orbit. Too Little Sideways V = Crash Too Much = Escape to Infinity

4. Circular Orbit circle implies Kepler’s Law

5. Example Calculate the period of a planet that is the same distance from the star Sirius that the Earth is from the Sun. r=1.5x10 11 m G=6.7x10 -11 M=4x10 30 kg About 9 Months

6. Non-Circular Formula Same Formula for P Use half orbit length for r 2r Focus Slow Fast

7. Escape Velocity Fall from Large Height Same Energy Needed to Reverse and Fly Away R Escape velocity is the speed at which object must be thrown upward to escape and never come down.

8. Example Earth R=6x10 6 m M=5x10 24 kg

9. Surface Gravity Force of Gravity at Surface of Body is So or The acceleration of a body near the surface. Independent of mass. Remember Galileo and the Leaning Tower of Pisa

10. Gravity on Earth 1 gee is 9.8m/s per second

11. Spin Up When a body is spinning “Angular Momentum” is conserved For spherical body: As R decreases P decreases Spin up of ice skater. Same for Collapsing Stars

12. Escape Velocity Fall from Large Height Same Energy Needed to Reverse and Fly Away R Escape velocity is the speed at which object must be thrown upward to escape and never come down. G=6.7x10 -11 in mks units

13. Question What is the escape velocity of the Moon? M=7.35x10 22 kg, R=1738km a)2000km/s b)200km/s c)20km/s d)2km/s e)0.2km/s

14. Answer: D Moon R=1700km M=7x10 22 kg

15. The Sun

16. Falls into “Disk Stability” 99.9% Ended in Sun (0.1% in Jupiter) Probably the Same Around All Stars -- Planets are Common Shoots Planet-Size Bullets into Space

17. Most of Mass Forms Ball in Center A Star Is a Hot Ball of Hydrogen (plus 11% Helium) One Million Miles

18. Question Why is the Sun spherical while the planets orbit in a disk? A) They actually orbit in a sphere B) The planets hit each other if they don’t all lie in the same plane C) The Sun has lost spin over time, while planet orbits cannot D) The Sun will eventually evolve to a disk

19. Answer: C The surface of the Sun is orbiting at far less than orbital speed. (just like the Earth) Back when it was first forming it spun much faster and was closer to a disk.

20. What Stops the Fall? Gravity Gets Stronger As Material Gets More Dense R smaller implies F greater The smaller it gets, the faster it falls in! Why doesn’t it just become a black hole? Or worse yet, a point-like singularity of mass?