Newtons First Law of MotionInertia

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Chapter 4

• Newtons First Law of MotionInertia

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4.1 Aristotle in Motion

• Greek scientist 4th century BC
• Studied motion and divided it into 2 types
• Natural Motion
• motion that is not caused by forces
• Violent Motion
• motion that is caused by forces, imposed motion

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4.1 Aristotle in Motion

• Natural Motion
• Objects seek their NATURAL resting place
• Heavy objects fall to the ground
• Light objects rise to the heavens
• Circular motion is natural for the heavens
• Believed that planets and stars moved in perfect
  circles around the Earth

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4.1 Aristotle in Motion

• Violent Motion
• The result of forces that push or pull
• A cart moves because it is pulled by a horse
• Caused by an external force
• Objects in their natural resting place cannot
  move themselves

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Aristotle in Motion

• Aristotles beliefs were common for over 2000
  years
• It was commonly believed that the proper state of
  an object is at rest
• Objects do not move, unless they are moving
  towards their natural resting place OR are being
  pushed or pulled
• Believed that the Earth was at rest

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Copernicus and the Moving Earth

• Nicolaus Copernicus (1473-1543)
• Theory of Moving Earth
• Reasoned that the Earth (and other planets) move
  around the sun.
• This was controversialit contradicted Aristotle.
• He kept his research a secret to escape
  persecution

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Galileo in Motion

• Supported Copernicus
• suffered house arrest due to his support
• Demolished the notion that a force is necessary
  to keep an object moving
• Force - any push or pull
• the SI unit of force is the newton (N kgm/s2)
• Friction - the force that acts between materials
  that touch as they moved past each other

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Forces

• A force exerted on an object can usually change
  an objects velocity with respect to time
• cause a stationary object to move
• throwing a ball
• cause a moving object to stop
• catching a ball
• can cause a moving object to change direction
• hitting a pitched baseball with a bat

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Forces

• Forces can act through contact or at a distance
• Contact forces result in a physical contact
  between two objects
• catching a football pulling a wagon
• Field forces do not involve in a physical contact
  between two objects
• gravitational force--when an object falls to the
  Earth, the object is accelerated by Earths
  gravity (Earth is exerting a force on the object
  with no immediate contact)

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Forces and Friction

• Friction is caused by irregularities in the
  surfaces that objects are touching
• friction acts on each of two objects which are in
  contact when one object moves or when there is an
  attempt to move it relative to the other
• these forces are parallel to the surfaces in
  contact and they act in directions to oppose
  motion
• smooth surfaces have less friction than rough
  surfaces
• If friction were absent an object would not need
  force to keep moving

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Forces and Friction

• Galileos ideas were supported by a variety of
  experiments
• A ball rolling down an inclined plane gains speed
  due to gravity
• A ball rolling up an inclined plane loses speed
  due to gravity
• On a level surface a ball would not roll with or
  against gravity, therefore speed should remain
  constant
• Galileo found that a ball moving on a smooth
  horizontal plane has almost constant velocity if
  friction were entirely absent, a ball moving
  horizontally would move forever

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Force and Friction

• Galileo also experimented with two inclined
  planes to support his reasoning

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Forces and Friction
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Newtons Law of Inertia

• The tendency for a body to keep moving is natural
  and every object resists change to its state of
  motion
• INERTIA is the property of a body to resist
  change
• the tendency of a body not to accelerate
• Newtons First Law
• An object at rest remains at rest, and an object
  in motion continues in motion with a constant
  velocity (that is, constant speed in a straight
  line) unless the object experiences a net
  external force
• http//www.physicsclassroom.com/mmedia/newtlaws/mb
  .gif

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Mass--a Measure of Inertia

• Suppose you kick an object.
• What makes an object easy to kick?
• Mass
• the lighter the object, the easier it is to kick
• the heavier the object, the harder it is to kick
• The amount of inertia an object has depends on
  its mass
• a brick has much more inertia than a piece of
  paper--because the brick has a much greater mass
  than the piece of paper

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Mass and Volume

• Mass is not volume
• volume is the measure of space an object takes
  up the units for volume are cubic meter, cubic
  inches, cubic feet, etc
• mass is measured in kg
• Mass does not depend on volume
• think of a bowling ball and a volleyball
• they have similar volumes, but the mass of a
  bowling ball is much greater than the mass of a
  volleyball

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Mass and Weight

• Mass is not weight
• weight is determined by the force of gravity on
  an object
• mass is the quantity of matter in an object
• Mass and weight are proportional to each other in
  a given place--must be the same place like Earth
• Weight mg
• Your weight on Earth is different than your
  weight on the moon g 9.8 m/s2 on earth and g
  1.6 m/s2

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Mass, Weight, Volume

• Does a 5 kg steel ball have 5 times as much
  inertia as a 1 kg block of steel?
• Yes--inertia mass and they have the same mass
• 5 times as much volume?
• Yes--both are made of steel
• 5 times as much weight, when weighed in the same
  location?
• Yes--they have the same mass and weight is
  proportional to mass

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Mass, Weight Volume

• Does a 5 kg steel block have five times as much
  inertia as a 1 kg piece of paper?
• Yes--inertia mass
• 5 times as much volume?
• No--steel and paper are different materials
  steel is much more dense than paper the volume
  of the paper would be much larger than the volume
  of the steel
• 5 times as much weight?
• Yes--they have the same mass and weight is
  proportional to mass

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Mass Weight

• One kilogram (kg) weighs 9.8 newtons (2.2 pounds)
• Felicia has a mass of 45.0 kg.
• What is Felicias weight on Earth?
• What is Felicias mass on Jupiter, where the
  acceleration due to gravity is 25.0 m/s2?
• What is Felicias weight on Jupiter?

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Free Body Diagrams

• Force is a vector
• depends on both magnitude and direction
• Diagrams used to show forces acting upon a single
  object are called free body diagrams
• the tail of the vector is attached to the object
  on which the force is acting
• the force vector points in the direction of the
  force
• the force vectors length is proportional to the
  magnitude of the force
• diagram will only show forces acting on one
  object (the object of interest)

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Free Body Diagrams
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Free Body Diagrams

• Draw free body diagrams for the following
  scenarios
• A boat travels upstream with a constant velocity.
• A box is pushed across the floor with a rightward
  acceleration.
• A box sits at rest on an inclined plane.

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Net Force

• Net Force - the vector sum of all forces acting
  on an object
• the unbalanced force
• the resultant of all forces acting on a system
• You push an object with a force of 5N at the
  same time your friend pushes the object, in the
  same direction, with a force of 7N. What is the
  net force?
• You push on the top of an object with a force of
  5N at the same time your friend pushed on the
  bottom of the same object with a force of 15N.
  What is the net force?

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Net Force

• 3. The wind exerts a force of 452 N north on a
  sailboat, while the water exerts a force of 325 N
  west on the sailboat. Find the magnitude and
  direction of the net force on the sailboat.
• http//www.physicsclassroom.com/mmedia/vectors/rb.
  gif
• 4. Derek leaves his physics book on top of a
  drawing table that in inclined at a 35 angle.
  The force of the table on the book is 18N. The
  force of gravity on the book is 22N. The force
  of friction on the book is 11N directly up the
  ramp. Find the net force on the book.

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Net Force

• Find the magnitudes missing forces for each
  object in the free body diagrams below

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Equilibrium

• Equilibrium--when net force equals zero
• objects that are at rest or are moving with a
  constant velocity (objects that are not
  accelerating) are in equilibrium
• Think of a 5 kg book sitting on a desk
• the book is at rest with respect to the table,
  therefore it is in equilibrium
• the force of gravity on the book is (5kg)(10m/s2)
  50 N down
• in order to be in equilibrium, the force of the
  table pushing back up on the book must be equal
  to 50 N up
• if something were between the book and the table,
  then it would feel like it was being smashed from
  both sides

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Equilibrium

• The force of the desk pushing back up on the book
  is called the normal or support force
• A car is traveling west with a constant velocity
  of 20 m/s, what is the net force on the car?
• If a car is accelerating downhill under a net
  force of 3674 N, what additional force would
  cause the car to have a constant velocity?

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Equilibrium

• Suppose you are hanging from a rope
• atoms are being stretched apart--this is called
  tension in the rope
• Tension is a force (unitnewton)
• How much tension is in the rope when you hang
  from it?
• If you are in equilibrium, the tension must equal
  your weight
• Suppose you hang on a bar from 2
  ropes--neglecting the weight of the bar--what is
  the tension in one rope?
• Each rope supports 1/2 of your weight therefore
  the tension must equal 1/2 your weight

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Equilibrium

• Statics - the study of forces in equilibrium
• Flip, an exhausted gymnast, hangs from a bar by
  both arms in an effort to catch his breath. If
  Flip has a mass of 65.0 kg, what is the tension
  in each of Flips arms as he hangs in place?

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Equilibrium

• 2. Jack and Jill lift upward on a 1.3 kg pail
  of water, with Jack exerting a force of 7.0 N and
  Jill exerting a force of 11 N. Jills force is
  exerted at an angle of 28 with the vertical. At
  what angle with respect to the vertical should
  Jack exert a force if the pail is to accelerate
  straight upward?
• Michelle likes to swing on a tire tied to a tree
  branch in her yard. If Michelle and the tire
  have a combined mass of 82.5 kg and Elwin pulls
  Michelle back far enough to make an angle of
  30.0 with the vertical, what is the tension in
  the rope?

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Vector Addition of Forces

• Why can you hang on a clothesline when it is
  suspended vertically, but not when it is
  suspended horizontally?
• Vertically Horizontally
• Tension weight Tension much greater than
  weight

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The Moving Earth Again

• Copernicus proposed the idea of the moving Earth
  in the 16th century
• the Earth is actually moving 30 km/s
• we do not feel this movement because everything
  is moving with the Earth--even things suspended
  above the surface of the Earth
• Suppose you jump vertically next to the wall on
  the west side of the room
• since the Earth is moving so fast--you should
  slam into the wall
• but you land right where you took off
• although you changed your vertical motion your
  horizontal motion remained at 30 km/s due to its
  horizontal inertia
• think of throwing something up in the air in a
  car that is traveling at a constant speed--the
  object will fall back to its original position