What Causes Motion? (Newton

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What Causes Motion?(Newtons Laws)

• IDS Physics Unit 02

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Some Initial Questions .

• Why do trains always have the right of way at
  road crossings?
• Why does a baseball hurt more than a foam ball if
  they both hit your face?
• Why did some hypothetical kid get in trouble
  for messing with his sister?

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Inertia

• Inertia is the property of matter that resists
  any changes in its state of motion.
• The inertia of an object is proportional to the
  amount of matter in the object
• Matter, and thus inertia, is measured by the
  mass.
• In the metric (SI) system, we use kg to measure
  the amount of matter.
• In the imperial (English) system, the unit of
  matter is the slug.

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Isaac Newton

• Sir Isaac Newton (1642-1727), mathematician and
  physicist, was one of the foremost scientific
  intellects of all time.
• Developed the calculus, theories of matter,
  motion, force, light and color, heating and
  cooling and many other ideas.
• Formulated the Three Laws of Motion

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Newtons First Law

• All objects in motion will remain in motion, in a
  straight line, unless an external force acts on
  them.
• All objects at rest will remain at rest unless an
  external force acts on them
• Demo The Dirty Tablecloth

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Law of Inertia

• Newtons First Law is also known as the Law of
  Inertia.
• change in its state of motion acceleration
• If there is no external force, there can be no
  acceleration.
• Objects will continue their current motion
  forever, unless something interacts with it.
• Examples toilet paper and tailgates

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Law of Inertia

• Example carrying a cup of tea
• Any change in the state of motion of the tray
  will spill the tea

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The Effect of Forces

• What if there is a force?
• the object will change its state of motion
  (accelerate)
• What does the acceleration depend on?

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Lab Connection
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Lab Connection

• In lab, we found the following relationships

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Combined Result of Lab

• If we combine the results of our lab, we get the
  relationship
• With the right units, we can write
• The acceleration of an object is directly
  proportional to the force on it and inversely
  proportional to its inertia (mass)

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So, whats a Force

• Any interaction between two physical objects can
  be considered a force.

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Types of Forces

• Force at a distance (field forces)
• Gravity
• Magnetism
• Electricity
• Contact forces
• Tension
• Applied Forces
• Friction

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Units for Force

• Rearrange our equation

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The Newton

• We will define a unit for force called the Newton

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Whats a Newton?

• Which of these will exert a downwards force of 1
  N on your hand?
• Kiwi
• Banana
• Cantaloupe
• Watermelon

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A handy Conversion

• Imperial Unit of Force the pound
• SI Unit of Force N
• 1.000 lbs 4.448 N
• Ballpark 1 N is about ¼ of a pound

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A problem

• But do all forces cause objects to accelerate?
• Only net, external forces cause accelerations.
• How do we find the net force?

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FBDs

• Free Body Diagrams are used to draw the forces on
  an object.
• All forces are vectors
• They have a magnitude
• And a direction
• We will represent them with arrows.

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Net Forces

• The net force on an object is the vector sum of
  all of the forces acting on it.
• The net force is also known at the resulting
  force.
• The summation notation SF will be used to denote
  the net force
• SF F1 F2 F3
• In one dimension, vector addition simply means
  that we need to account for the sign of the
  vector, which represents its direction.

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Finding SF

• A force of 50 N pulls on a wagon to the left.
• A force of 35 N pulls on a wagon to the right.
• Make a Free Body Diagram and find the net force.

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Newtons 2nd Law

• We can now revise our statement of this law

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Example Problem (1)

• Cameron pulls on a 25 kg. wagon with a force of
  10 N to the left. Jacob2 pulls to the right with
  a force of 15 N. What is the acceleration of the
  wagon?

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Example Problem (2)

• The engine exerts a force of 10,000 N forward on
  a 500 kg. boat. The boat accelerates at 2 m/s2.
  What is the backwards force of the water on the
  boat?

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Example Problem (3)

• A rope exerts a force of 300. N on sled, which
  accelerates forward at 1.5 m/s2. There is also
  present a friction force of 50.0 N, which acts
  opposite the tension. What is the mass of the
  sled?

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Gravity A Special Force

• Gravity exerts a force on each of us called our
  weight.
• This force causes every mass to accelerate at a
  rate of -9.807 m/s2.

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Weight

• We can calculate this force with Newtons Second
  Law

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Example (3)

• Find the weight, which is the force that gravity
  exerts on an object on earth that has 60. kg of
  matter.

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Falling Objects

• Why do they fall at the same rate if the force is
  twice as big on the 1 kg weight?
• The bigger mass has a greater inertia, so the
  greater force meets a greater resistance to
  changes in the state of motion.
• Similar Problem Why does a truck not accelerate
  faster than a car if it has a greater force
  pushing it forwards?

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Newtons 3rd Law

• Every interaction involves two objects. The
  forces on these two objects are equal in
  magnitude and opposite in direction
• Every action has an equal and opposite reaction
  is an equivalent but archaic statement of the 3rd
  law.

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Forces come in pairs

• Identify the force pairs
• A hammer exerts a force on a nail
• Earth pulls downwards on a person
• A car pushes backwards on the road

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Example

• A bug collides with the window of a speeding bus.
• The force of the bus on the bug is the same as
  the force of the bug on the bus.
• The effects of the force are not equal, however.
• What is the last thing that goes through a bugs
  mind as he collides with a car windshield?

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Game

• Time to play the third law game .