Forces _ Newtons Laws by maclaren1

VIEWS: 272 PAGES: 53

									              Forces & Newton’s Laws
   •    Identifying and defining “forces”
   •    Free Body Diagrams
   •    Gravitational Forces
   •    Mass and weight
   •    The Law of Inertia
   •    The Law of Acceleration
   •    The Law of Force-Counterforce
   •    Normal force, Tension, and friction
   •     The vector nature of forces
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              So, what is
              a “force”,
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              So, what is a “force”
              Forces cause cars to move!

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              So, what is a “force”?

              Forces cause planes to fly!

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              So, what is a “force”?
                    Forces cause planets

                      to orbit the sun!
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              So, what is a “force”?


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              So, what is a “force”
    Defn: a force is a push or a pull that
     causes a change in motion, shape, or

   Iftwo forces are equal and opposite they
    are called balanced forces.
   More than one force can combine to make
    a net force (total force).
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              Think About It...
                  Think of all the different
                  kinds of forces in the
                  universe... List as many
                  of them as you can.
                  How many different
                  forces are there?
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              The Forces of Nature
    I. Gravitational:
    •attraction bet. masses
    •tides, gravity, weight    II. Weak Nuclear:
                               •helps to explain
  II. Electromagnetic:          atomic collisions
  •friction        •tension
  •adhesion        •lift       III. Strong Nuclear:
  •electrostatic   •drag       •binds atomic nuclei
  •buoyant         •magnetic

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              Representing Forces...
    Forces       are vectors
        – Forces are drawn as arrows (vectors)
        – forces add like vectors.
        – the sum of all the forces is called the net
   A     picture of a body with arrows drawn
       representing all the forces acting upon it
       is called a FREE BODY DIAGRAM.

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              Try it...
              Draw a picture of your
                book sitting on the
               desk. Identify all the
                forces acting on it.

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              Free Body Diagrams...
                          T (table)


                          W (weight)

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              Free Body Diagrams...
          What forces are acting on a
          skier as she races down a hill?

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              The Answer...


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              The Answer...
                              f and d


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              Gravity and Weight...
    • The force pulling on your mass
    (toward the ground) is called your
   W           =mxg
                            1 Newton
    What     are the       (N) = 1
        units of weight            2
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              Guess Some Common
                         Weight               Mass
                    SI (N)   Eng (lb)   SI (kg) Eng (slug)
   a car
   a bowling ball
   a quarter
   a bus

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              How’d you do?

                          Weight                Mass
                     SI (N)   Eng (lb)   SI (kg) Eng (slug)
   you                   667       150        68      4.66
   a car              8,896      2,000       907     62.17
   a bowling ball       71.2      16.0        7.3     0.50
   a quarter          0.139      0.031        0.0     0.00
   a bus            177,920    40,000    18,141      1,243

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              As the skydiver falls what
              are the magnitudes of the
              forces acting on him?

                           QuickTime™ and a
                          GIF decompressor
                    are neede d to see this picture.

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              Apparent Weight
    •Is your weight always the same on the
       •How about in an elevator?

          •What is this
                                  QuickTime™ and a
          sky diver’s          Cinepak decompressor
                          are needed to see this picture.
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              Apparent Weight
    •When accelerating up…
                                 Your rate of
                  WA = mg + ma   acceleratio

    •When accelerating down…

                  WA = mg - ma
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              Apparent Weight Example
               On the Mantis (at Cedar Point) at the
                bottom of the first hill you experience an
                acceleration of 27 m/s/s (about 3 g‟s). What
                is your apparent weight at that point?
               What is your apparent weight at the top of
                the 2nd hill when you experience an
                acceleration of 2.2 m/s/s?

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              Free Fall
     •In the absence of air
     resistance ALL objects
     accelerate at 9.81              QuickTime™ and a
                                     GIF dec ompres sor
                              are needed to see this picture.


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              True or False
    1. The elephant and the feather each experience
    the same force of gravity.
    2. The elephant experiences a greater force of
    gravity, yet both the elephant and the feather
    have the same mass.
    3. On earth, all objects (whether an elephant or
    a feather) have the same acceleration.
    4. The elephant clearly has more mass than the
    feather, yet they each weigh the same.
    5. The elephant has the greatest acceleration,
    yet the amount of gravity is the same for each.
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              Think about it...
    What does “gravity” depend on?
     If you‟re standing on a scale and the
      pressure in the room rises what
     If the Earth stopped spinning how
      would your weight change?
     So, what is gravity caused by?

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              Law of Universal Gravitation

          • Any two bodies with mass
          will attract eachother with a
                gravitational force!
               • More mass ==> more force
               • farther away ==> less force

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                Fg  G 2
              G  6.67 10
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   Find   the force of attraction between
      you and your neighbor (estimate
      distances and masses).

   Find    the force of gravitational
      attraction between the Earth and
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              Think about it...
   When   a baseball is hit, what causes it to
    slow down? What prevents it from
    slowing down?
   When your car stops fast, why do you
    lunge forward?
   Under low friction circumstances which
    is harder to start moving, a large object or
    a small object?
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       Def: An object’s tendency to
       resist a change in motion is called
       its inertia.intrinsic property that depends only on
        inertia is an
              – more mass --> more inertia!

                                     QuickTime™ an d a
                                     GIF decompressor
                               are need ed to see this p icture .

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              The Law of Inertia...
      An object will stay in a state of rest
      or straight line motion unless acted
      on by an unbalanced force.

       an       inertial reference frame is a f.o.r.
              in which the law of inertia holds

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              IRF’s or not???
              a  car moving w/ constant velocity
               the Earth
               a bus that is slowing down
               a skier going down a hill
               a person on a merry-go-round
               a skateboard rolling on a level

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              Think about it...
    A little girl sits on a skate
    board and her friend
    pulls her with a constant
    force. Describe the
    motion of the girl.

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              The Law of Acceleration
   When an unbalanced force is applied to an
   object it will accelerate in the direction of the
   net force with an acceleration proportional to
   the force applied and inversely prop. to the

                     ∑F = m x a
        If there is only one force then the equation is
         simply: F=m x a

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     What  force is needed to decelerate a
      2,000 lb car from 35 mph to rest in 5
     ~2900N

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    A   jet thruster applies a force of 200,000N
      at maximum burn. If the jet has a mass of
      15,500kg what is the acceleration of the
      jet? How long will it take to increase
      speed from 100 m/s to 150 m/s?
     a ~ 13 m/s2 and t ~ 4sec!!

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              Think About it...
  A  textbook rests on a table. What forces
    act on the book? On the table?
   True or False. When you jump the Earth
   An 8 ton bus crashes into a 1200 lb VW
    Rabbit. If the bus applies a force of
    20,000 lb on the car, what is the force of
    the car on the bus?
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              The Law of Force-Counterforce
    When one body exerts a force on
    another body the second one exerts
    an equal and opposite force on the
         unaccompanied     forces do not exist in
         „action-reaction‟ forces are not the same
          as „balanced‟ forces.
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              Questions to consider...
       Identify the “counterforce”…
          – The push of a rifle on your shoulder when you
            shoot a gun.
          – The hit of the lineman on the QB.
          – The backwards force of the tires on the road.
          – The gravity of the Earth on the Moon.
       Why can you push harder on the pedals when
        pulling up on the handle bars?

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              Think About It...
     A   feather rests on a strong, solid oak
       table. Does the table exert any forces
       on the feather?
      I lean on a solid cement wall. Does it
       exert forces on me?
      What force causes a ball to bounce?

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              Normal Forces...
    when  you stand on the ground your weight
     pushes down on it. It pushes up on you
     with the same force! That‟s called a
     normal force.
    a normal force is always perpendicular to
     the surface in contact.
    Take a minute and identify all the normal
     forces in the room.
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              Think about it...
    What    is the reading on the scales in
       the two diagrams below?
          fig. 1               fig. 2

                   (weights are 200 N each)
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   Tension is a force that pulls things apart.
   Tension is an electromagnetic force.
   Tension is the force that pulls in BOTH

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              Tension…A Deeper Analysis
     Massless ropes and things...

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              Think about it...
  an ice skater coasts to a stop...
  a marble rolls across the table and comes to
  A boy in the back of a truck stays still while the
   truck accelerates.
  a skydiver reaches terminal velocity...
  What force is responsible for the above?

  If         you said FRICTION, you are correct!
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      So what exactly is friction and what
       does it depend on?
      Friction is a resistive force:
              – surface locking
              – electrostatic attraction
              – air drag
              – fluid viscosity

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     In      mechanics,
    µ  (Greek “mu”) is a constant which is
      different for each situation.
     N is the normal force
          – What is “N” for a book sliding across the
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              Types of friction...
        Sliding        Friction (kinetic)
                             fk = µk x N
        Static       Friction (static)
                            fs = µs x N
             In each case the only difference is the
              coefficient of friction, µ.

                             µk ≤ µs
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              Determining µ...
      To determine the coefficient of friction
       between two surfaces just pull a block at a
       constant speed across the table top with a
       spring scale. What force is necessary to do
      Draw a Free Body Diagram. How can you
       find µ (this is for kinetic friction)?
      How is it possible to find the coefficient of
       static friction?

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      What is the force of friction when a
       150 lb boy (bike weight included) applies the
       brakes w/o skidding while riding down the
       street on his bike (µs=0.9)? How would it
       change if he locked up his brakes (µk=0.7)?
      How many meters will the boy above take to
       come to rest in each situation if his initial
       speed is 26 mph?

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              Forces are vectors
      Forces can be resolved into vertical and
       horizontal components.
      Ex. A boy pushes on a lawn mower with a
       force of 400 N. If the mower weighs 65 lbs
       and the handle makes an angle of 45° with the
       ground what is the horizontal acceleration
       (ignore friction)? How would the problem
       change if you don‟t ignore friction?

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              Lawnmower Problem
              Fx  Fcos(45)
              Fx  400cos(45)  283N
              Now, W  mg and 65lb  289N
              So, 289N  m(9.81)
                 m  29.5kg
                   Fx   283N
              ax            9.59 m / s2
                   m 29.5kg
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              Wrap-Up Questions
      What are the four basic forces of the universe? Which
       is the strongest?
      What is inertia? How does it relate to Newton‟s first
      What is equilibrium?
      What kind of motion do forces cause?
      How does the force of your car‟s windshield on a
       mosquito compare to the force of the mosquito on the
       wind shield?
      What is a tension? Normal force? Friction?

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