Modern Astronomy

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Modern Astronomy

• Johannes Kepler was the first astronomer to
  correctly determine the shape of the planets
  orbits.
• Isaac Newton, the father of modern physics, was
  able to explain why objects in space moved as
  they did.
• The work of Kepler and Newton is the foundation
  of our understanding of astronomy.

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Keplers Laws

• Keplers First Law all planets orbit the sun in
  the shape of an ______.
• Keplers Second Law planets move faster when
  they are ______ to the sun.
• Keplers Third Law the length of a planets ____
  is proportional to its average _______ from the
  sun.

ellipse
closer
orbit
distance
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Beyond Kepler

• Keplers three laws were based on observations,
  not physics or mathematical formulas.
• Kepler didnt know why his laws worked, he just
  knew that they did.
• It was Sir Isaac Newton who finally provided
  theories to explain why Keplers Laws worked.

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Early Astronomy
? Sir Isaac Newton
English scientist Sir Isaac Newton (16421727)
explained gravity as the force that holds planets
in orbit around the Sun.
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Gravitys Influence on Orbits
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Early Astronomy
? Sir Isaac Newton
Although others had theorized the existence of
gravitational force, Newton was the first to
formulate and test the law of universal
gravitation. The universal law of gravitation,
helped explain the motions of planets in the
solar system.
? Universal Gravitation
Gravitational force decreases with distance.
The greater the mass of an object, the greater
is its gravitational force.
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Gravity

• Gravity is the force that
• holds us to the Earth
• causes a rock to fall towards the ground
• causes the Earth to go around the Sun
• causes the Sun to be pulled towards the center of
  the Milky Way galaxy
• Gravity acts between any two objects even if they
  are far apart.
• action at a distance

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Newtons Laws of Motion

• 1st Law
• A body at rest, or in uniform motion, will remain
  so unless acted upon by an unbalanced force.
• 2nd Law
• The change in motion (acceleration) is
  proportional to the unbalanced force
• 3rd Law
• For every action there is an equal and opposite
  reaction

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Newtons Laws and Astronomy

• 1st Law Law of Inertia
• Inertia the tendency of an object to keep moving
  (or stay still) unless another force acts on it
• How is this important to astronomy?
• The planets keep moving in their orbits. They
  would keep going in a straight line due to
  inertia, but the suns gravity pulls them in.

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Newtons Laws and Astronomy

• 2nd Law Fma
• Usually written as an equation, it shows us that
  the force of an object is related to its mass and
  how much it is accelerating
• How is this important to astronomy?
• Since force is related to mass and acceleration,
  Newtons second law helps explain the orbits of
  space objects.
• A comet passing by a star (with a large mass)
  will actually acceleratesame with planets in the
  solar system
• Keplers Second Law

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Newtons Laws and Astronomy

• 3rd Law Law of Action/Reaction Forces
• When you push on an object, you feel pressure on
  your hand because it pushes back on you (with an
  equal and opposite force)
• How is this important to astronomy?
• We wouldnt be able to accurately calculate
  measurements for space flight without this law.
• A rocket launch is a perfect example the action
  force is the fuel exiting the rocket, the
  reaction force is the rocket pushing off of the
  ignited fuel

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Summary

• Keplers and Galileos Laws provided Newton with
  important clues that helped him formulate his
  laws of motion
• Newton arrived at 3 laws that govern the motion
  of objects
• The law of inertia
• The law of force
• The law of action and reaction
• Newton also arrived at a law of gravity
• But it seemed to require action at a distance!