Newtons Laws of Motion

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Newtons Laws of Motion
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• Violent motion resulted from pushing or pulling
  of an outside force.
• Natural motion the nature of an objects motion
  depends on its combination of earth, water, air
  and fire.
• Air moves up
• Earth moves down
• Feather air earth, slowly moves down
• Conclusion - An objects proper state of motion
  was at rest.
• The earth must be at rest what force is large
  enough to push the earth?

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Aristotelian Thinking

• If the earth orbited the sun it would move at 30
  km/sec.
• If a bird descended from the tree in one second
  the worm would have been swept by the moving
  earth 30 km!

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Galileo Galilei (1564 1642)Gave credence to
the Copernican sun-centered model and discredited
Aristotelian ideas. He proved force was not
needed to keep an object in motion.
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Galileo imagined motion without friction. Objects
have a resistance to change their state of
motion. He called this resistance inertia.
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Galileos Interpretation

• If the earth orbited the sun it would move at 30
  km/sec.
• If a bird descended from the tree in one second
  the bird, the worm, the tree and everything else
  would move 30 km in that second.

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• Aristotle failed to imagine what motion would be
  like without friction.
• He did not recognize friction for what it is,
  namely a force.
• This impeded the progress of physics for nearly
  1800 years.

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• Newtons first law is a restatement of the
  concept of inertia.
• Every object continues in its state of rest, or
  of uniform motion in a straight line, unless it
  is compelled to change that state by a force
  impressed upon it.

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Asteroids have been moving though space for
billions of years. What keeps them moving?
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The amount of inertia an object has depends on
its mass

• Mass is not volume
• Mass represents kilograms of matter while volume
  represents liters of space.
• Mass is not weight
• The astronaut finds it just as difficult to shake
  the anvil in its weightless state as it is to
  shake it in its weighted state on earth.

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Newtons Second Law
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Gravity accelerates objects by the same amount
regardless of their mass.Why dont we normally
observe this?
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Newtons Third Law
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Newtons Third Law For any force there is an
equal and opposite force
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A force is not a thing in itself but makes up an
interaction between one thing and another. You
cant push on the wall unless the wall pushes
back on you. A single force can not exist. Force
must come in pairs.
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Identify the action and reaction
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Worksheets

• Action reaction
• Interaction

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An interaction has an equal and opposite force
pairAn interaction does not always create an
equal and opposite acceleration.
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Rifle and BulletWhich experiences more force?
Which experiences more acceleration?
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Action Windshield hits bugReaction

• Which object experiences more force?
• Which object experiences more deceleration?

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Interactions between objects do not have to be in
contact with each other
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• If the Earth exerts a force on the Moon does the
  Moon exert a force on the Earth?

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Gravitational force used to keep planets in
circular motion
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A force is required to keep an object moving in a
circle (orbit)
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Ancients believed celestial objects were divine.
No force needed to keep them moving in circles

• Newton recognized a force must act on celestial
  objects to keep them in orbit.
• If an apple hanging in a tree is pulled to Earth
  then the Moon must also be pulled to Earth.
• Gravity extends out into the heavens.

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Section 5.4 Types of Forces and Free Body Diagrams
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Worksheet on Identifying Forces