Physics 55 Monday, October 3, 2005

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Physics 55Monday, October 3, 2005

• Orbital speed law, rotation curves, and the
  discovery of dark matter.
• Tides, tidal forces, and the Roche limit.

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Fun Physics NC State Fair, Oct 14-13
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The Orbital Speed Law for Distributed Masses
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Rotation Curves (p. 615 of text)
Rigid rotation
Large central mass
Amount of mass increases with r
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Existence of Dark Matter
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Tides Differential Gravitational Forces
Why are there two tides a day?
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Relative Pull of Moon and Sun on Oceans
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Spring Versus Neap Tides
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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 Moons orbit
is circular to high accuracy.
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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.
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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
• Once a month.
• Once every two months.
• Once a year.
• Twice a year.
• Some other amount of time.
• I dont know how to answer this.

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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.
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Why Black Holes Are Dangerous