1. Physics 55 Monday, October 3, 2005 1.Orbital speed law, rotation curves, and the discovery of dark matter. 2.Tides, tidal forces, and the Roche limit.

2. Fun Physics: NC State Fair, Oct 14-13

3. The Orbital Speed Law for Distributed Masses

4. Rotation Curves (p. 615 of text) Rigid rotationLarge central mass Amount of mass increases with r

5. Existence of Dark Matter

6. Tides: Differential Gravitational Forces Why are there two tides a day?

7. Relative Pull of Moon and Sun on Oceans

8. Spring Versus Neap Tides

9. Tides Not Caused By Elliptical Orbit of Moon A common misunderstanding is the belief that tides are caused by Moon becoming closer to Earth as part of its elliptical orbit, and so pulls harder on the water that is closer to the Moon creating a tide. But this is not consistent with fact that high tide occurs twice per day, also not consistent with fact that Moon’s orbit is circular to high accuracy.

10. Tidal Forces Occur With Galaxies Galaxy NGC 2207 on left passing by NGC 2163 on right. These galaxies lie 114,000,000 ly from the Sun, in Canis Major.

11. PRS Question: Tides of Future Earth Assume that in the far future, Moon will take two months to orbit once around Earth, while Earth will slow down to rotate once a year. Then a tide will be observed: 1.Once a month. 2.Once every two months. 3.Once a year. 4.Twice a year. 5.Some other amount of time. 6.I don’t know how to answer this.

12. Tidal Stresses and the Roche Limit Brown spots mark the places where fragments of Comet Shoemaker-Levy 9 tore through Jupiter's atmosphere in July 1994. Because of its retrograde orbit, Triton, the biggest moon of Neptune, is spiraling in toward Neptune and will eventually cross the Roche limit and disintegrate, producing a bright ring system.

13. Why Black Holes Are Dangerous