How Things Move

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Chapter 3

• How Things Move
• Galileo asks the right Questions

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Chapter 3

• Aristotles physics
• Agrees with most peoples commonsense feelings
  about motion
• Its a wrong view to take and requires a closer
  look to see why and where Aristotles ideas of
  motion are wrong.
• His ideas will be discarded by Newton in his
  attempt to fundamentally explain motion.

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Chapter 3

• Aristotle
• He noticed that some motions maintain themselves
  without assistance while others could be
  maintained only by some outside agent.
• Boiled motion into two categories
• Natural Motion
• Violent Motion

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Chapter 3

• Aristotle contd
• Natural Motion
• Motion that could maintain itself
• Examples Solid objects falling, liquids falling
  or running downhill, air rising, and flames
  leaping.
• The four natural motions occur on Earth because
  everything is made of four different
  substances--earth, water, air, and fire--and each
  of these substances are striving to reach their
  natural place.

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Chapter 3

• Aristotle contd
• He believed that all objects were composed of
  these substances, and that an objects motion was
  determined by how much of each element the object
  contained.
• For instance,
• Wood floats because its mostly made up of air
• Hot air raise because its mostly made up of fire
• Page 67 in the book lists more examples

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Chapter 3

• Aristotle contd
• One thing to keep in mind is that all the motion
  discussed thus far is directly downward or
  upward. What about horizontal motion?
• Violent Motion
• Motion that can only be maintained by an outside
  agent.
• Examples Pushing a boulder off a cliff, or
  pulling a wagon down the road
• Motion that mostly occurs horizontally.

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Chapter 3

• Aristotle contd
• Ultimately, Aristotle felt that all motion on
  Earth was a combination of violent and natural
  motion.
• To explain the motion of the heavenly bodies,
  I.e. the planets and stars, Aristotle claimed
  that they were made of a 5th element called ether.

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Chapter 3

• From Aristotle to Galileo
• Although Aristotelian physics explains a lot of
  different types of motion, the theory does have
  its weakness.
• Difficulties with Aristotelian physics

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Chapter 3

• From Aristotle to Galileo
• Galileo (1564-1642)
• First recognizable person to object with
  Aristotelian physics which had permeatted
  philosophy and culture until 1600 A.D.
• Supported Copernican view of the universe
• Can be thought of as the Grandfather of
  experimental physics.

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Chapter 3

• Galileo contd
• 1st person we recognize as practicing the
  scientific method
• Performed experiments where he rolled balls up
  and down inclines with varying smoothness
• This lead him to the conclusion that, in the
  absence of friction, a ball that started rolling
  on a horizontal surface would roll forever.

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Chapter 3

• Galileo contd
• His methods included the following
• Experiments were designed to test specific
  hypotheses.
• Idealizations of real-world conditions were made
  to eliminate any side effects that might obscure
  main effects.
• Limited the scope of inquiring by considering
  only one question at a time.
• Established quantitative methods to measure the
  motion of bodies, speed of falling objects, etc.

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Chapter 3

• Inertia
• Is the tendency of an object in motion to stay in
  motion and an object at rest to stay at rest.
• Idea first purposed by French philosopher and
  scientist Rene Descarte.
• What happens to objects in the absence of
  gravity?
• All objects have inertia
• Inertia doesnt really explain anything, rather
  is a word that stands for the unexplainable fact
  that unassisted objects do keep moving.

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Chapter 3

• The Law of Inertia
• A body that is subject to no external influences
  will stay at rest if it was at rest to begin with
  and will keep moving if it was moving to begin
  with in the latter case, its motion will be in a
  straight line at an unchanging speed. In other
  words, all bodies have inertia.
• A body that is subject to no external forces must
  be unaccelerated. (most concise form)

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Chapter 3

• More on Inertia
• Space is a good place to test motion without air
  resistance.
• Planets keep moving around the sun due to their
  inertia rather than some mystical force pushing.
• Inertia doesnt provide the answer to why the
  planets travel around the sun in near circular
  orbits.
• Book lists other examples on page 72 through 74.

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Chapter 3

• Motion
• Rather than keep talking about motion in a
  qualitative, its useful to talk about motion in a
  quantitative way.
• Scientists like to specify their measurements
  with the aid of a few basic quantities. The
  basic quantities useful in motion is distance,
  time, and direction.

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Chapter 3

• Average Speed
• Defined as the distance traveled divided by the
  traveling time.
• A cars speedometer gives you an idea of your
  speed. The one thing it doesnt tell you is what
  direction you are heading.
• The concept speed is used to indicate the speed
  at every instance along an objects path.
• Velocity is a term used to indicate speed and
  direction.

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Chapter 3

• Speed
• Equation s D / t
• s speed
• has units of meters per second or m/s
• D distance
• has units of meters or m
• t time
• has units of seconds

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Chapter 3

• Acceleration
• Defined as the change in speed divided by the
  time it takes to make that change.
• Like Velocity, acceleration has a directional
  significance associated with it.
• The number part of acceleration is referred to as
  the acceleration.

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Chapter 3

• Galileos Law of Falling
• If air resistance is negligible, then any two
  objects that are dropped together will fall
  together, regardless of their weights and their
  shapes, and regardless of the substances of which
  they are made.
• The problem with this law is that it seems to
  contradict our own common experience.

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Chapter 3

• Galileos Law of Falling contd
• Experience tells you that if you drop a bowling
  ball and a feather that bowling ball will hit the
  ground.
• The reason is that the feather experiences much
  more air resistance than the bowling ball.
• This experiment was performed by the astronauts
  on their first voyage to the moon.

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Chapter 3

• Galileos Law of Falling contd
• The astronauts found that the bowling ball and a
  feather fall to the ground at the same rate and
  hit the ground at the same time.
• The rate at which an objects falls to the ground
  is called the acceleration due to gravity.
• Free fall is motion that is influenced by gravity
  alone. Skydivers undergo free fall when they
  jump out of a perfectly good airplane.