Do Now:

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Do Now

• What is a force?

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NewtonsLaws of Motion
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General Overview

• Three physical laws that describe relationships
  between forces acting on a body and that bodys
  motion.
• Formulated by Sir Isaac Newton, English scientist
  and mathematician, in 1687.
• Have stood the test of timeOnly two
  exceptions
• Very fast objects explained by Einsteins
  Special Theory of Relativity
• Very small objects explained by Theory of
  Quantum Mechanics

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The Three Laws

• First Law of Motion
• An object will maintain its current state of rest
  or uniform velocity unless acted upon by an
  unbalanced force
• Second Law of Motion
• The acceleration of an object is directly
  proportional to the net force exerted upon it and
  inversely proportional to its mass
• Third Law of Motion
• For every action there is an equal and opposite
  reaction

WAY TOO MANY UNFAMILIAR TERMS!!!
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Press the Easy Button!

• First Law of Motion inertia
• Second Law of Motion Fnet m?a
• Third Law of Motion action-reaction

YOU MIGHT STILL NEED A VOCAB LIST!!!
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The Vocab Breakdown

• Force a push or pull on an object
• Rest not moving
• Velocity speed in a given direction
• Acceleration change in velocity
• Uniform constant, not changing

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The Vocab Breakdown

• Net overall sum, considering different
  directions
• Mass amount of matter in an object
• Directly Proportional as the independent
  variable increases, the dependent variable
  increases
• Inversely Proportional as the independent
  variable increases, the dependent variable
  decreases

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So Newton used the force too?!?
NO!!! NOT THAT KIND OF FORCE!
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Types of Forces

• Applied Force (Fapp) force which is applied to
  an object by another object or by a person
• Tension (Ftens) force which is transmitted
  through a string, rope, or wire when it is pulled
  tight by forces acting at each end.
• Normal Force (Fnorm) support force exerted upon
  an object which is in contact with another stable
  object. Exerted perpendicular to the surface.

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Types of Forces

• Friction (Ffric) force exerted by a surface as
  an object moves across it or makes an effort to
  move across it. Opposes the motion of the
  object.
• Air Resistance (Fair) special type of frictional
  force which acts upon objects as they travel
  through the air. Opposes the motion of the
  object.
• Gravitational Force (Fgrav) force with which the
  earth, moon, or other massive body attracts an
  object towards itself. By definition, this is the
  weight of the object.

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Newtons First Law of Motion

• An object will maintain its current state of rest
  or uniform velocity unless acted upon by an
  unbalanced force.
• What it meansIn the absence of an unbalanced
  force
• an object at rest will remain at rest
• an object in motion will remain in motion at the
  same speed and in the same direction

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1st Law Examples Auto Collisions
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1st Law Examples Auto Collisions

• What is the occupants state of motion before the
  collision?
• Constant forward velocity
• What is the occupants state of motion after the
  collision without a seatbelt?
• Constant forward velocity
• What does a seatbelt provide?
• An unbalanced force on the occupant
• It decelerates the driver to rest

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1st Law Examples Whiplash

• Suppose you are sitting at a stoplight when you
  are rear-ended by another car.
• To an onlooker on the sidewalk, what initially
  happens to your head?
• It stays at rest as the car and your body move
  forward.
• It was at rest and will remain at rest.
• An unbalanced force does not act on it until
• the headrest pushes your head forward.

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1st Law ExamplesThe Tablecloth Parlor Trick

• TRY THIS AT HOME... with PAPER plates!!!
• Why does it work?
• If the tablecloth is pulled quickly enough, the
  frictional force is minimal, so the table setting
  remains practically at rest as the tablecloth is
  pulled out from underneath.

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Inertia

• The First Law is often called the Law of Inertia
• Inertia is the tendency of an object to maintain
  its state of rest or constant velocity
• Inertia is really a measure of MASS
• More mass ?Greater tendency to stay at rest or
  in motion
• Less mass ?Less tendency to stay at rest or in
  motion
• Which is easier to move from rest a pebble or a
  boulder? Which is easier to stop when moving?

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So inertia is just weight?!?

• Not quite, young grasshopper!
• Mass and weight are not the same thing!
• Mass amount of matter in an object
• Measured in kilograms (kg) in metric system
• Measured in slugs in English system
• Never changes!
• Weight force of gravity on an object
• Measured in Newtons (N) in metric system
• Measured in pounds (lbs) in English system
• Changes based on location (altitude planet)

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Calculating Weight

• Weight can be calculated very easily from the
  mass of the object.weight mass x acceleration
  due to gravityOn Earth, acceleration due to
  gravity is 9.8 m/s2weight mass x 9.8 m/s2

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Calculating Weight

• Solved Example What is the weight of a 50 kg
  high school student?weight mass x 9.8
  m/s2 equationweight 50 kg x 9.8 m/s2 plug
  chugweight 490 kg?m/s2weight 490
  N answerNote 1 N 1 kg?m/s2

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Calculating Weight

• Try this one on your own! Show all work!
• What is the weight of 4 kg sack of potatoes?
• Solutionweight mass x 9.8 m/s2 equationweight
  4 kg x 9.8 m/s2 plug chugweight 39.2
  kg?m/s2weight 39.2 N answer

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Newtons Second Law of Motion

• The acceleration of an object is directly
  proportional to the net force exerted upon it and
  inversely proportional to its mass.
• as net force increases, acceleration
  increases(as long as mass is constant)Example
• Just you pushes a heavy piano. Piano doesnt
  speed up quickly.
• Both you and a partner push the piano. Piano
  speeds up quicker.

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Newtons Second Law of Motion

• The acceleration of an object is directly
  proportional to the net force exerted upon it and
  inversely proportional to its mass.
• as mass increases, acceleration decreases(as
  long as force is constant)Example
• You pull an empty wagon with all your force.
  Wagon speeds up quickly.
• You pull a wagon loaded with bricks with all your
  force. Wagon speeds up slowly.

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Newtons Second Law of Motion

• The 2nd Law is expressed as an equation Fnet
  mawhere Fnet is the net force acting on
  the object is measured in newtons (N)
• m is the mass of the object is measured in
  kilograms (kg) a is the acceleration of the
  object is measured in meters per second squared
  (m/s2)

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Whats a Net Force?

• Sum of forces acting in different directions
• Lefts vs. Rights Ups vs. Downs
• How do we know if theres a net force?The
  objects MOTION will tell us!!!
• At rest No net force
• Moving at constant velocity No net force
• Accelerating Net force acts

The easiest way to analyze forces and determine
the net force is by drawing!!!
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Drawing Forces

• We need to diagram both the magnitude and
  direction of these forces
• Magnitude size, amount, how much
• Direction which way is the push or pull
• A force ALWAYS needs both magnitude and direction
• INCORRECT a force of 5 N
• CORRECT a force of 5 N to the right

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Drawing Forces

• To draw forces, we use arrows
• These arrows are called force vectors
• Length of vector magnitude of force
• Direction of vector direction of force

F
5 N right
10 N left
3 N down
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Free Body Diagrams

• To diagram ALL the forces acting on an object, we
  use a FREE BODY DIAGRAM
• Box represents the object
• Arrows represent forces
• Arrows drawn outward from box in direction of
  force
• Arrows are labeled with name of force

Fapp
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Free Body Diagrams

• To draw correct FBDs, you need to think about the
  forces acting on the object.
• This is difficult at first.
• Practice makes perfect!
• Key to success determine if theres a net force
• Remember
• At rest No net force
• Moving at constant velocity No net force
• Accelerating Net force acts

Net force is always in the direction of the
acceleration!
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Free Body Diagrams

• Example What does the FBD for a physics book
  resting on the desk look like?
• SolutionIs there a net force? No, the book is
  at rest.Consider all forces and decide which
  ones act.
• Gravity? YES! Gravity always acts pulls down.
• Applied? NO! Nobody is pushing the book.
• Tension? NO! Theres no string, rope, etc.
• Friction? NO! It isnt sliding or trying to
  slide.
• Normal? YES! The desk pushes up on the book.
• So GRAVITY pulls down NORMAL FORCE pushes up.
  Together it looks like

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Fnorm
BUT, PRACTICE MAKES PERFECT
Fgrav
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Determining Net Force from FBDs

• Add forces in same direction
• Subtract forces in opposite directions
• The VECTOR SUM is the NET FORCE

5 N
6 N
1 N
2 N
(Net Force)
(All Forces)
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Determining Net Force from FBDs

• Treat horizontal and vertical forces separately.

(Net Force)
(All Forces)
3 N
1 N
4 N
1 N
5 N
Trigonometry is needed to determine a single net
force from these two!!!
2 N
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So, what about Fnet ma?

• Working with this equation is a synthesis of 2nd
  Law knowledge
• First, identify forces acting and draw FBD
• If forces are balanced, Fnet 0
• If forces are unbalanced, express Fnet as either
• A single force
• Vector sum (combination) of of two or more forces
• Second, identify given info from problem
• Third, Plug Chug (you may need to rearrange!)
• Finally, express your answer in the correct
  units!!!

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2nd Law Example Problem 1

• Using a rope, you pull your kid brother on a sled
  across the ice with a force of 75 N. If your
  brother and the sled have a combined mass of 30
  kg, what is the resulting acceleration of the
  sled? (Ignore friction)

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2nd Law Example Problem 1

• Identify forces
• Vertically Gravity Normal balance out
• Horizontally Tension only!!!
• Net Force? Yes!!! Fnet Ften
• Identify info
• Ften 75 N m 30 kg a ?
• Plug ChugFnet ma75 N (30 kg)a (need to
  rearrange!)a 75 N / 30 kg
• Answera 2.5 m/s2

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2nd Law Example Problem 2

• You pull your kid brother in wagon with a force
  of 90 N. The resulting acceleration is only 2
  m/s2. If your brother and the wagon have a
  combined mass of 30 kg, what is the force of
  friction acting on the wagon?

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2nd Law Example Problem 2

• Identify forces
• Vertically Gravity Normal balance out
• Horizontally Applied Friction
• Net Force? Fnet Fapp - Ffric
• Identify info
• Fapp 90 N m 30 kg a 2 m/s2 Ffric ?
• Plug ChugFnet ma90 N Ffric (30 kg)(2
  m/s2)90 N Ffric 60 NFfric 90 N 60
  NFfric 30 N
• AnswerFfric 30 N