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Forces _ Newtons Laws

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					              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
SHS Physics                                   1
              So, what is
              a “force”,
              anyway?
SHS Physics                 2
              So, what is a “force”
              Forces cause cars to move!




SHS Physics                                3
              So, what is a “force”?




              Forces cause planes to fly!

SHS Physics                                 4
              So, what is a “force”?
                    Forces cause planets




                      to orbit the sun!
SHS Physics                                5
              So, what is a “force”?

         Forces
         hold
         atoms
         together!

SHS Physics                            6
              So, what is a “force”
    Defn: a force is a push or a pull that
     causes a change in motion, shape, or
     direction.

   Iftwo forces are equal and opposite they
    are called balanced forces.
   More than one force can combine to make
    a net force (total force).
SHS Physics                                    7
              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?
SHS Physics                                    8
              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


SHS Physics                                           9
              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
          force.
   A     picture of a body with arrows drawn
       representing all the forces acting upon it
       is called a FREE BODY DIAGRAM.

SHS Physics                                             10
              Try it...
              Draw a picture of your
                book sitting on the
               desk. Identify all the
                forces acting on it.

SHS Physics                             11
              Free Body Diagrams...
                          T (table)

                           Book

                          W (weight)


SHS Physics                            12
              Free Body Diagrams...
          What forces are acting on a
          skier as she races down a hill?




SHS Physics                                 13
              The Answer...
                         FN

                              d&f




                              W
SHS Physics                         14
              The Answer...
                         FN
                              f and d




                        W

SHS Physics                             15
              Gravity and Weight...
    • The force pulling on your mass
    (toward the ground) is called your
    weight.
   W           =mxg
                            1 Newton
    What     are the       (N) = 1
        units of weight            2
        (force)?
                            kg·m/s
SHS Physics                              16
              Guess Some Common
              Masses/Weights
                         Weight               Mass
                    SI (N)   Eng (lb)   SI (kg) Eng (slug)
   you
   a car
   a bowling ball
   a quarter
   a bus


SHS Physics                                                  17
              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




SHS Physics                                                   18
              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.




SHS Physics                                            19
              Apparent Weight
    •Is your weight always the same on the
    Earth?
       •How about in an elevator?

          •What is this
                                  QuickTime™ and a
          sky diver’s          Cinepak decompressor
                          are needed to see this picture.
          apparent
          weight?
SHS Physics                                                 20
              Apparent Weight
    •When accelerating up…
                                 Your rate of
                  WA = mg + ma   acceleratio
                                 n!


    •When accelerating down…

                  WA = mg - ma
SHS Physics                                21
              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?


SHS Physics                                                    22
              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.


     m/s/s!




SHS Physics                                                     23
              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.
SHS Physics                                            24
              Think about it...
    What does “gravity” depend on?
     If you‟re standing on a scale and the
      pressure in the room rises what
      happens?
     If the Earth stopped spinning how
      would your weight change?
     So, what is gravity caused by?


SHS Physics                                   25
              Law of Universal Gravitation

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

SHS Physics                                    26
                      m1m2
                Fg  G 2
                       r
                                      2
                               Nm
              G  6.67 10
                         11
                                  2
                                kg
SHS Physics                               27
              Examples
   Find   the force of attraction between
      you and your neighbor (estimate
      distances and masses).

   Find    the force of gravitational
      attraction between the Earth and
      you.
SHS Physics                                  28
              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?
SHS Physics                                        29
              Inertia...
       Def: An object’s tendency to
       resist a change in motion is called
       its inertia.intrinsic property that depends only on
        inertia is an
              mass
              – more mass --> more inertia!

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




SHS Physics                                                         30
              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
              true.

SHS Physics                                             31
              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
                sidewalk?

SHS Physics                                         32
              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.

SHS Physics                         33
              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
   mass.

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

SHS Physics                                                34
              Examples
     What  force is needed to decelerate a
      2,000 lb car from 35 mph to rest in 5
      seconds?
     ~2900N




SHS Physics                                   35
              Examples
    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!!




SHS Physics                                         36
              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
    accelerates.
   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?
SHS Physics                                      37
              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
    first.
         unaccompanied     forces do not exist in
          nature.
         „action-reaction‟ forces are not the same
          as „balanced‟ forces.
SHS Physics                                           38
              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?


SHS Physics                                                 39
              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?




SHS Physics                                      40
              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.
SHS Physics                                      41
              Think about it...
    What    is the reading on the scales in
       the two diagrams below?
          fig. 1               fig. 2




                   (weights are 200 N each)
SHS Physics                                    42
              Tension...
   Tension is a force that pulls things apart.
   Tension is an electromagnetic force.
   Tension is the force that pulls in BOTH
    directions




SHS Physics                                       43
              Tension…A Deeper Analysis
     Massless ropes and things...




SHS Physics                               44
              Think about it...
  an ice skater coasts to a stop...
  a marble rolls across the table and comes to
   rest…
  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!
SHS Physics                                             45
              Brainstorm...
      So what exactly is friction and what
       does it depend on?
      Friction is a resistive force:
              – surface locking
              – electrostatic attraction
              – air drag
              – fluid viscosity

SHS Physics                                   46
              Friction...
     In      mechanics,
                             f=µxN
    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
            floor?
SHS Physics                                             47
              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
SHS Physics                                             48
              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
       this?
      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?

SHS Physics                                            49
              Examples
      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?


SHS Physics                                            50
              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?


SHS Physics                                            51
              Lawnmower Problem
              Fx  Fcos(45)
              Fx  400cos(45)  283N
              Now, W  mg and 65lb  289N
              So, 289N  m(9.81)
                 m  29.5kg
              Finally,
                   Fx   283N
              ax            9.59 m / s2
                   m 29.5kg
SHS Physics                                  52
              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
       Law?
      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?

SHS Physics                                                    53

				
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