9 Gravity

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9 Gravity

• Intensity
• Universal Law of Gravity
• The Moon
• Tidal Forces and Tides
• Gravity inside a Planet
• Einsteins Theory of Gravity
• Black Holes

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Intensity

• intensity is the power delivered per unit area
• SI Unit watts/m2
• Example the brightness of this screen is
  determined by the intensity of visible light
  radiation falling on it.

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Intensity Calculation

• intensity from a point source follows an
  inverse-square law dependence with distance

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Gravity follows the Intensity Rule
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Universal Law of Gravity

• all matter is weakly attracted
• the force is an inverse-square law
• where G 6.67x10-11 Nm2/kg2
• Example Two 100kg persons stand 1.0m apart

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The Earth-Moon gravitational attraction keeps the
system together.
The Moon is falling toward the Earth, but since
it is moving it always misses.
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From 4 million miles away on December 16, 1992,
NASA's robot spacecraft Galileo took this picture
of the Earth-moon system. The bright, sunlit half
of the Earth contrasts strongly with the darker
subdued colors of the moon. Our moon is one of
the largest moons in the solar system. It is even
larger than the planet Pluto. In this picture,
the Earth-moon system actually appears to be a
double planet.
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Orbital Lock Moon with Earth

• caused by gravitational torque
• same side of moon always faces earth
• there is no dark side of the moon. all parts of
  moon are illuminated during a month.
• there is a far side of the moon. this side
  never faces earth.

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Moons Orbit is inclined about 5 degrees to
Earths. Eclipses occur when the moon crosses the
Earths orbit in line with Sun and Earth.
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Eclipses
Solar Eclipse
Total solar eclipses are rare events. Although
they occur somewhere on Earth approximately every
18 months, it has been estimated that they recur
at any given place only once every 370 years, on
average (Stephenson, p.54).
Lunar Eclipse occurs when moon is on opposite
side of Earth.
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Gravity causes differential (rather than
constant) pull
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Differential Pull Causes Tides
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Spring Tides (largest)
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Neap Tides (smallest)
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Gravity inside and outside Earth.
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Einstein modeled gravity as stretched space-time
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Black Hole caused by very small and massive
object.
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Wormhole, path to another universe?
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Condensation of Matter
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End