Welcome to Physics I !!!

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Physics I95.141LECTURE 69/22/10
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Exam Prep Problem

• A rock is launched with an initial speed vo22m/s
  at an angle ?o65º from the origin. Ignore air
  resistance.
• A)(10pts) Draw a coordinate system for this
  problem and show the trajectory of the rock.
  Give the initial velocity vector of the rock in
  component form.
• B) (8pts) How high does the rock go and how long
  does it take the rock to get to that height?
• C) (7pts) How far, horizontally, will the rock
  travel before it lands?

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Exam Prep Problem

• A rock is launched with an initial speed vo22m/s
  at an angle ?o65º from the origin. Ignore air
  resistance.
• A)(10pts) Draw a coordinate system for this
  problem and show the trajectory of the rock.
  Give the initial velocity vector of the rock in
  component form.

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Exam Prep Problem

• A rock is launched with an initial speed vo22m/s
  at an angle ?o65º from the origin. Ignore air
  resistance.
• B) (8pts) How high does the rock go and how long
  does it take the rock to get to that height?

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Exam Prep Problem

• A rock is launched with an initial speed vo22m/s
  at an angle ?o65º from the origin. Ignore air
  resistance.
• B) (7pts) How far, horizontally, will the rock
  travel before it lands?

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Administrative Notes

• HW review Session Tonight
• 630 pm, OH 218
• Structure of HW Review Sessions
• Why we do it this way
• Why it is important to try
• Structure of this course
• Combination of your effort/our effort
• Lecture Notes
• Practice exams/prep problems
• Attendance Policy

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Outline

• Force
• Newtons 1st Law
• Newtons 2nd Law
• Newtons 3rd Law
• What do we know?
• Units
• Kinematic equations
• Freely falling objects
• Vectors
• Kinematics Vectors Vector Kinematics
• Relative motion
• Projectile motion

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Force

• Intuitively, we understand a Force to be a push
  or pull on an object
• This could be a racket hitting a ball, elevator
  cable pulling on a car, a locomotive pulling a
  train
• These are all contact forces, where the force is
  exerted by one object in contact with another
• Force can also be exerted by magnets, electric
  fields, or gravity
• Here, Force is exerted by a field, not an object,
  but this is still a force!
• Force has magnitude and direction it is a VECTOR!

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Force

• In order for something at rest to start moving, a
  Force must be exerted on it.
• However, if something is already moving, you also
  need a Force acting on it to stop motion!

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Inertia

• Obviously, some things are harder to get moving
  than others
• If you had to choose whether to carry an ice
  skater or an offensive lineman up to the Deans
  office on the 5th floor which do you think would
  be easier?
• We quantify inertia with a measure we call mass
• SI units mass ? kg

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Newtons 2nd Law

• If Newtons first law tells us how an object
  behave with no Force, we also want to understand
  how an object behaves with a Force applied.
• The acceleration of an object is directly
  proportional to the net force applied to the
  object, and the constant of proportionality is
  mass.

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Units for Force

• Using Newtons 2nd Law, we can figure out the
  units of Force.

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Redefine Force

• An action capable of accelerating an object
• Acceleration is a vector, as is Force. Net
  acceleration is in the same direction as net
  Force.

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Example Problem

• A 1500kg car traveling at 100km/h needs to stop
  in 55m. What force is required?

• Draw diagram

• Determine coordinate system

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Example Problem

• A 1500kg car traveling at 100km/h needs to stop
  in 55m. What force is required?

• Knowns/Unknowns (UNITS)

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Example Problem

• A 1500kg car traveling at 100km/h needs to stop
  in 55m. What force is required?

• Choose equations and solve

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Connection between Newtons 1st, 2nd Laws

• If Fma
• And no Force is exerted on an object
• How can we describe the objects velocity?

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Newtons 2nd Law

• Do Newtons 1st and 2nd Law work every where?
• Say you are in a pickup truck that is
  accelerating, and you put a frictionless block in
  the back.
• What do you see happening to the block?

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Newtons 2nd Law

• Do Newtons 1st and 2nd Law work every where?
• Say you are in a pickup truck that is
  accelerating, and you put a frictionless block in
  the back.
• What is the Force causing this?

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Newtons 2nd Law

• Do Newtons 1st and 2nd Law work every where?
• Say you are in a pickup truck that is
  accelerating, and you put a frictionless block in
  the back.
• What does someone on the ground see?

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Inertial Reference Frames

• Newtons Laws only work in Inertial reference
  frames
• Inertial reference frame
• One in which Newtons Laws hold
• A Reference Frame that moves with a constant
  velocity
• A Reference Frame at rest

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Newtons 3rd Law of Motion

• 1st Law Law of Inertia, things want to keep on
  doing what they are already doing.
• 2nd Law Forces cause objects to accelerate, with
  aF/m.
• But where do forces come from?
• Forces come from other objects
• Hammer exerts a force on the nail
• The Earth exerts a force on a falling ball
  (gravity)
• Elevator cable exerts force on car

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Newtons 3rd Law

• But when hammer hits nail, it stops very quickly.
• This is deceleration, which means the nail must
  also exert a Force on the hammer.
• This is Newtons 3rd Law
• Whenever one object exerts a Force on a second
  object, the second object exerts an equal Force
  in the opposite direction.
• Sometimes, you hear For every action, there is
  an equal and opposite reaction
• Remember though, the action and reaction act on
  different objects!

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Action/Reaction

• For every Force exerted on an object, there is an
  equal and opposite force exerted by that object

• They actually get the Physics right here!

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Describing Forces

• When we talk about Forces, it is very important
  that we describe the Force as exerted by
  something and acting on another thing.
• For instance, in the case of the ice skater,
  force is exerted on the skater by the ice.
• For the car, the Force making the car move is
  exerted by the ground on the tires.
• When we start solving problems, it is important
  to keep this straight. If you are interested in
  the motion of an object, you only want to sum up
  the Forces acting on that object.
• For a person walking, the ground is what moves
  the person forward.

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The Motivator
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Draw Diagram

• Want F, need to figure out acceleration
• Assume
• Tf0.25s
• M60kg
• ?x1.8m
• ?y-1m

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Projectile Motion Problem (I)

• What is initial launch velocity?

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Acceleration

• If we know the initial launch velocity, then we
  know the final velocity after acceleration
• Initial velocity of part II final velocity of
  part I

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What Did We Learn

• Newtons Laws
• Law of Inertia
• Fma
• Action/Reaction
• Solving basic Force problems
• How to describe motion of an object with Force
  applied.