Warm Up 510.25.11

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Warm Up 5 10.25.11

• What causes motion? In other words, what causes
  things to start, stop and change directions?
• Sorry, computer issues have made me use a
  powerpoint from the web

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• Dynamics
• - the study of what causes motion

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• Aristotle (384-322 BCE) Greek philosopher who
  arrived at several principles of nature.
• Greeks did not use experimentation because they
  believe that the senses can be deceived- the
  correct way to do science is to think about it.
• He proposed the 2 Types of Motion
• a. Natural Motion
• b. Violent Motion

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Natural Motion

• Occurs when objects fall to rest on the surface
  of the earth or when they are sailed up into the
  sky, like a puff or smoke. Objects seeking their
  natural place in the world.

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Violent Motion

• Occurs when a moving object is moved away from
  its natural place, such as picking up a bottle
  from the ground or throwing the ball into the
  air.
• The object in violent motion must be kept in
  motion by a violent force or it will come to
  rest

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Aristotle vs Galileo

• Aristotles views were widely held for centuries.
• It wasnt until Galileo came along that his
  theories were completely disproved.

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6 Notions of Force and Motion

• 1.) If there is motion there is force acting.
• 2.) There cannot be a force without motion- if
  there is no motion then there is no force acting.
• 3. When an object moving there is a force in the
  direction of motion.

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6 Notions of Force and Motion

• 4.) A moving object has a force within it which
  keeps it moving.
• 5.) A moving object stops when its force is used
  up.
• 6.) Motion is proportional to force acting.
  Therefore a constant speed result from a constant
  force.

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• Answers?
• NOT one of these notions are correct.
• These are 6 commonly held misconceptions of force
  and motion, and they all originate form
  Aristotles views.

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• Galileo (1564- 1642)
• Conceived of motion as continuous.
• Conceived of motion on a frictionless surface
• An object once placed in motion , will move
  further until another force brings it to stop.
• Implied that the natural state of objects is
  continuous motion.

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Galileos Breakthrough

• He realized that things slowed down due to
  friction .
• He called it RESISTANCE
• In order to understand force and motion, you need
  to envision motion without friction.

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INERTIA

• Objects on frictionless surface has it.
• Objects will never stop, will go forever.
• Objects needs no force to keep it moving
• Newton incorporated this into his work and called
  the property inertia.

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Newstons First Law

• The Law of Inertia
• When no external , unbalanced force acts on an
  object its velocity remains constant. or
• If no net force acts on an object, it maintains
  its state of rest or its constant speed in a
  straight line or
• An object at rest will remain at rest, an object
  in motion will remain in motion in a straight
  line unless either are acted upon by an external
  force.

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Implications

• External force is required to change the velocity
  of an object.
• External force must be unbalanced.
• Helps explain the behavior of objects in motion-
  they resist change in motion due to their inertia.

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Mass a measure of inertia

• Inertia is directly proportional to the mass of
  the object.
• A large object at rest is hard to move.
• A large object having constant velocity is hard
  to stop.
• Mass is defined as the amount of matter in an
  object. In the book it is defined as the
  laziness of an object.

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Example

• An elephant at rest has a lot of laziness so it
  has a greater tendency to remain at rest.
• (hard to move)

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Example

• A runaway train at constant velocity has a lot of
  laziness so it has a greater tendency to stay
  in motion.
• (hard to stop)

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Differentiating Mass and Weight

• A given quantity of mass will remain the same
  wherever it is in this physical world.
• The weight depends on the mass of an object and
  it is equal to the force of gravity on an object.

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Examples

• If you have a mass of 75 kilograms, your mass on
  earth, moon and Jupiter will be 75 kilograms.
• Your weight depends on the force of gravity
  exerted on you by the other mass (earth, moon and
  Jupiter).

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MASS and WEIGHT are DIRECTLY PROPORTIONAL TO
EACH OTHER.

• More mass means more weight.
• Conversion of your weight in pounds to your mass
  in kilograms.
• 2.2 pounds 1 kilogram

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MASS and WEIGHT are DIRECTLY PROPORTIONAL TO EACH
OTHER.

• If your weight is 50 lbs, what is your mass in
  kg? 50 lbs ( 1 kg/ 2.2 lbs) 22.73 kg
• The earth is pulling you with a force of 50 lbs
  towards its center. Also equal to a force of 490
  newtons.
• Do you want your weight
• to be in newtons?
• 50 lbs 490 newtons

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NET Force resultant force.

• Which object will remain at rest, according to
  Newtons First Law?
• Which object will continue moving with constant
  velocity, according to Newtons First Law?

8 N
5 N
5 N
5 N
The cat will move
5 N
5 N
5 N
8 N
This car will accelerate.
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Light Bulb Moment

• No matter how many applied forces are acting on
  an object AND the net force is equal to zero, the
  object will maintain its inertia.
• Balanced forces on an object, maintains the state
  of inertia of an object.

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If the forces acting on an object are cancelled,
the object is said to be in a state of
equilibrium. Meaning the net force is equal to
zero and the object maintains its inertia.
Weight
Weight
Weight
Air Friction
Air Friction
Normal Force by the ground
Falling objects will experience terminal velocity
The object is at rest
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Example

• If you are standing, at rest, the balanced
  forces acting on you are
• a.) your weight and
• b.) the upward support force of the ground called
  the
• normal force.

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Questions

• 1. Why do professional photographers use heavy
  cameras for their shots?
• 2. While making a turn to the right, why is your
  body moving towards the left side of the car?
• 3. Why do we use safety seat belts?
• 4. In which location will it be difficult to
  shake a 50 kg rock, on earth or in space?

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Questions

• 5. Which object has more inertia, a 15 kilogram
  bowling ball or a 15 kilogram bag of pure cotton?
• 6. While standing still what is your inertia with
  
• respect to the sun? with respect to the ground?
• 7. What is the value of the normal force if you
  are standing on the table?
• 8. If you throw a ball horizontally and we
  neglect air friction and the force of gravity,
  how will you describe the motion of the ball?