Chapter 2 : Newtons First Law of Motion Inertia

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Chapter 2 Newtons First Law of Motion -
Inertia

• By April Senger

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What Will Happen?

• Have you ever seen the old pull the tablecloth
  out from under your dishes trick?
• Why does this work?
• Would it be better to use a heavier or lighter
  object?

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Aristotle (384-322 B.C.)

• Unnatural Motion
• He felt that these motions required applied
  forces
• Animals or people were responsible for Unnatural
  Motions such as a horse pulling a cart
• He believed that heavy objects fell faster than
  light ones ? Again!
• His ideas were considered fact for 2000 years!
• What were the issues that Aristotle didnt
  address?

• Natural Motion
• The Earth is directed either up or down
• He felt that boulders falling down and dust
  rising was natural because all objects moved
  towards their resting places
• Aristotle believed Natural Motion didnt need a
  force such as the orbits of our solar system ?

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Galileo and Inertia

• In the early 1500s an Italian Scientist
  challenged Aristotle's belief that heavy objects
  fell faster than light ones
• Galileo dropped many objects off of the Leaning
  Tower of Pisa

• Taking air friction into account, he decided they
  all fall at the same rate
• What was next?
• Galileo rolled balls up and down tiles at
  different angles?
• Why would he do this?

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The AnswerInertia!

• Galileo noticed that a ball rolling down a ramp
  picked up speed
• It slowed if it continued to roll up a ramp
• He decided that Gravity was responsible for
  causing the change in speed
• He also declared that if you moved a ball on a
  flat surface without frictionit would go forever!

• Objects didnt need a force to keep moving-only
  to get started
• Inertia was Born! It is the tendency of an object
  to stay as they are
• If you are moving you stay moving and if you are
  at rest you stay at rest

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Who is Right?

• If I am playing pool, the ball will stop moving
• How would Aristotle explain this?
• How would Galileo explain this?
• What do you think?

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Here Comes the Math

• Galileo expanded on his concept of why things
  move fast or slow
• He defined speed as the distance covered per
  unit of time
• Anytime you hear the work per DIVIDE
• Speed d/t
• What is the speed of a car traveling 20 miles in
  an hour?
• What is the speed of a baseball thrown 6 meters
  in 1 second?

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Speed vs. Velocity

• The difference isVelocity is speed WITH
  DIRECTION
• If I say the ball was thrown at 75 mi per hour is
  that speed or velocity?
• How can I change it to be the opposite?
• Vector quantities include the magnitude of an
  object and the direction
• Velocity is a vector quantity

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Constant Speed Velocity

• An object with constant speed is not slowing down
  or speeding up
• What is an object with constant velocity look
  like?
• Is a Ferris Wheel moving with constant speed,
  velocity or both?

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Lets Practice

• What is the average speed of a cheetah that
  sprints 100 meters in 4 seconds?
• How about if it sprints 50 meters in 2 seconds?
• The speedometer on a bicycle east reads 50 km/h.
  It passes another bicycle moving west at 50 km/h.
  Do both have the same speed? Do they both have
  the same velocity?
• She moves at a constant speed in a constant
  direction is too wordyHow can you state this
  more simply?

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Motion is Relative

• Relative is not a person in your familyIt means
  compared to
• For instance, I am tall relative to an Umpa Lumpa
• List as many things are you can that are not
  moving
• Our speed relative to the sun is 100,000 km/h
  and you are moving even faster towards the center
  of our galaxy

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More Relative Facts

• The space shuttle is moving 30,000 km/h relative
  to the Earth
• A car is traveling 60 mi/h. A fly in the backseat
  flies to the front of the car at 2 mi/h. How fast
  is the fly traveling?
• The Theories of Motion relative to our
  scientistsAristotle established the ideas of
  motion, Galileo used experiments rather than
  logic to show how things moved (not logical
  arguments about why), and finally we come to

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Sir Isaac Newton (1642-1727)

• Newton extended Galileos concept of inertia and
  developed two more laws to describe all motion
• Inertia if referred to as Newtons first law and
  was restated Every object continues in a state
  of rest, or in a state of motion in a straight
  line at constant speed, unless it is compelled to
  change that state by forces exerted upon it

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Concept Check

• If a space shuttle moves in a circular orbit
  around the Earth, is a force needed to maintain
  its high speed? If suddenly the force of gravity
  were turned off, what type of path would the
  shuttle follow?

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Forces

• Forces are vectors because they have magnitude
  and direction
• We draw them as an line with an arrow pointing in
  the direction of the net force
• Forces can be applied in many ways such as
  pushing, pulling, friction etc. and do not have
  to be from the same directions

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You Decide

• Both pull with a force of 5 N East 10 N East
• One pulls with 5 N East 5 N West 0 N
• One pulls with 5 N West 10 N East 5 N East

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Our Friend ? F

• ? means the sum of and F means forces
• If the pull up is 5 Newton's and the pull down is
  5 Newton's then the sum of forces is zero 5 -5
  0
• The pull up is positive and the pull down is
  negative
• Adding a negative is like subtracting
• This is called Mechanical Equilibrium

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Concept Check

• If you are hanging from some rings on the play
  ground and your hands are evenly dividing the
  weight between the rings, how would a scale
  readings in both the rings compare?
• Suppose she hangs with slightly more of her
  weight supported by the left ring. How would the
  scale on the right read?

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Heavy Thinking

• If a book is sitting on the table and gravity is
  pulling on it we should see motion
• It is not moving so there must be force pushing
  up with equal magnitude
• What is the force pushing up?
• This is called a supportive force or normal force
• What does ?F equal in this case?

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Weight a Minute

• When you stand on your scale to find out how much
  you weigh, what is the supportive or normal
  force?
• What determines the numbers on the scale?
• There are two forces involved. What are they?

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Concept Check

• What is the net force on a bathroom scale when a
  110-pound person stands on it?
• Suppose you stand on two bathroom scales with
  your weight evenly divided between them. What
  will each scale read? How about it you lean with
  more of your weight on one scale than the other?

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Equilibrium

• Equilibrium is a state of no change
• If a duck is swimming with a constant velocity
  across a lake is it in equilibrium?
• There are several kinds of equilibrium
• Static equilibrium is when the objects are at
  rest
• Dynamic equilibrium is when the object is moving
  with a constant velocity
• Others include topics such as thermal equilibrium

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Concept Check

• An airplane flies at a constant velocity. In
  other words, it is in equilibrium. Two horizontal
  forces act on the plane. One is the thrust of the
  propeller that pushes it forward. The other is
  the force of air resistance that acts in the
  opposite direction. Which forces is greater?
• Can you name any other forces acting on the
  plane?
• What about someone in a parachute?

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Fast Large Scale Objects

• The Earth is moving in a circle super fast right?
  So why is it that when you jump out of a tree
  that you land straight down? Shouldnt you land
  where the Earth was a little while ago?
• If you were riding in an airplane and flipped a
  coin, could you catch it?

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To Wrap It Up

• Aristotle didnt know about inertia and assumed
  that horizontal movement was unnatural and
  required a steadying force.
• Both Galileo and Newton saw that all objects
  followed the same rules. They believed that to
  keep moving no force was required if friction was
  absent