Gravitational Interactions

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Gravitational Interactions

• Chapter 13

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Gravitational Fields

• Areas of a certain distance away from a
  gravitational source experience the same field
  strength
• Like ripples in a pond after stone dropped in
• The field strength on the surface of the Earth is
  about 9.8 m/s2 or about 10 m/s2

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Going to the center of the Earth

• According to Newton, the closer you get to an
  object the stronger gravity is, but if you were
  travelling to the center of the Earth, all of the
  mass above you would start to cancel the gravity
  below you, so as you travel towards the center of
  earth gravity will become less and less strong

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Weight Weightlessness

• How you feel weight, is different than your
  actual weight
• As long as you are near the surface of the Earth
  you will always have the same weight but you may
  feel like you have a different weight
• This can happen if you are accelerating up or
  down
• Imagine an elevator..

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Ocean Tides From the Moon

• Side of the Earth closest to the moon, has a high
  tide
• As well as the side of earth furthest from the
  moon
• This happens because
• There is a large different in gravitational
  strength of the moon on either side of the Earth
• Gravity from the moon cancels out a little bit of
  Earths gravity
• Gravity from moon is of course stronger when moon
  is close
• And weaker when far away

Earth
Earth
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The Sun also causes Tides

• The Sun, like the Moon pulls on Earth, so the
  side of Earth closest to Sun, receives a high
  tide from Sun
• Tides from the Sun are weaker than Earth
• Why is this?
• Sun actually has a lot stronger pull on Earth
  than the moon does, so why doesnt it pull more??
• Not that big of a difference in how far the sun
  is from sunny side compared to dark side

8
Earth Tides and Atmosphere Tides

• Tides dont just happen with water
• Occur w/ the ground
• Earth is really just a thin solid crust on top of
  molten (liquid) rock
• Ground rises and lowers small amount depending on
  moons location
• and with the atmosphere
• Atmosphere has much larger tides than oceans do

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Satellite Motion

• Satellite - any object that orbits around
  another object includes, planets, moons,
  man-made satellites, etc.
• As discussed before, satellites are just
  projectiles that fall at the exact same rate that
  Earth (or another planet) curves away from it
  and never reaches the surface

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Newton (and everyone else) thought

• All satellites travel in perfectly circular
  orbits
• NOT TRUE
• When Newton figured out Universal Law of
  Gravitation, scientists should have been able to
  predict paths of planets, moons, etc.
• But Planets, moons, etc. Were not where they
  were supposed to be!
• Planets did not follow these predicted paths
• So something must be wrong
• Then along came Johannes Kepler

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Elliptical paths Kepler

• Kepler came up with Three Laws of Satellite
  Motion
• These laws explained that planets other
  satellites travel in ellipses (ovals), not
  circles
• When tested Keplers laws almost perfectly
  predicted paths of planets and moons
• This means planets are closer to the Sun at some
  points in their orbit, and farther from the Sun
  at other points

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Earths Orbit

• Very nearly circular, but elliptical like all
  other planets
• Elliptical path of Earth does not affect weather
  at all
• Other planets, like Mercury, have more
  exaggerated elliptical orbits

Orbit of Mercury
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Conservation of Energy and Satellite motion

• Based on the fact that KE PE always stays
  constant..
• Planets travel faster when they are closer to the
  Sun, and slower when they are further away

Slow
Fast
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Escape Speed

• Speed that something needs to travel in order to
  permanently escape an objects gravitational pull
  without getting pulled back
• Escape speed for the Earth is roughly 7 miles per
  sec.

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Einstein Gravity

• Keplers predictions of where objects should be
  were very close but still somewhat off
• Albert Einstein actually defined exactly how
  gravity works and his Theory of General
  Relativity predicted perfectly the orbits of all
  planets
• General Relativity stated mass warped Space-Time
  and this warping is what we feel as gravity
• Has been tested many times over the past 100
  years, and all tests confirm Einsteins
  predictions

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Black Holes

• Consequence of General Relativity
• What they are?
• Remnants of a collapsed star
• Shrunk to such a small size, that its gravity
  became infinitely strong. So strong its escape
  velocity is faster than the speed of light
• One of the most interesting objects in the
  Universe
• Nothing can escape a black hole
• Creates an infinitely deep hole in Space-Time,