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Mechanics Kinematics and Newtons Laws

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Mechanics Kinematics and Newtons Laws Powered By Docstoc
					Mechanics
A force is a push or pull.


          Contact Forces
Non-contact Forces
such as gravity,
magnetic, and
electric forces.
Newton’s First Law of Motion
   An object continues in a state of rest or in
   a state of motion at constant speed along
   a straight line…
             unless compelled to change that
             state by a net force.
                                      The mass of an
                                      object is a
                                      quantitative
                                      measure of
                                      inertia. The SI
                                      Unit of mass is
                                      kilogram (kg).
It’s Slinky!!
          Immediately after I release the spring,
          what does the bottom of the spring do?
     1.         It begins to fall immediately at the
                same rate as the rest of the
                                                                                      0%
                spring.
     2.         It begins to fall, but at a slower
                rate than the top of the spring.
     3.         It remains stationary.
     4.         It moves upward as the top of the
                spring moves downward.

                                                                                  1    2   3    4




1    2     3     4    5    6    7    8    9    10   11   12   13   14   15   16       17   18       19   20
21   22    23    24   25   26   27   28   29   30
Immediately after I release the spring,
what does the bottom of the spring do?
   Newton’s Second Law of Motion
When a net external force F acts on
an object of mass m, the acceleration
a that results is directly proportional to
the net force and has a magnitude

                                            F
that is inversely proportional to the
mass.
The direction of the acceleration is
                                         a
the same as the direction of the net
                                             m
force.

SI Unit of Force: kg • m/s2 = newton (N)
Individual Forces                 Net Force



4N           10 N                             6N


      The net force on an object is the
      vector sum of all forces acting on
      that object.
               m = 1850 kg




 F  + 275 N + 395 N  560 N = +110 N
      a
          F   110 N  0.059 m/s  2

           m          1850 kg
If the airplane’s mass is 13 300 kg, what
is the magnitude of the net force that the
catapult and jet engine exert on the
plane?
                    F  ma  (13 300 kg)(31m/s )
                                               2


                         = 4.1105 N
The Normal Force
The Normal Force & Friction
Static Frictional Force

  The magnitude of the static frictional force can have
  any value from zero up to some maximum value, fsmax,
  depending on the applied force.
  In equation form, we write:
                                  fs  f   s
                                            m ax


  The equality holds only when fs attains its maximum
  value, which is
                f   s
                     m ax
                              s FN
      where µs is the coefficient of static friction and FN
  is the magnitude of the normal force.
The Normal Force & Friction
 Static Frictional Force - Example

The Force Needed to Start a Sled
Moving

A sled is resting on a horizontal patch of
snow and the coefficient of static friction is
0.350. The sled and its rider have a total
mass of 38.0 kg. Determine the horizontal        fs       F
force needed to start the sled barely
moving.


  f   s
       max
               s FN   s mg
              (0.350)(38.0 kg)(9.80 m/s )            2


              130 N
Kinetic Frictional Force

The magnitude of the kinetic frictional force is given by

                        f k   k FN
where k is the coefficient of kinetic friction and FN is the
magnitude of the normal force.
Kinetic Frictional Force - Example




 Sled Riding

 A sled is traveling at 4.00 m/s along a horizontal stretch of snow. The
 coefficient of kinetic friction is 0.0500. How far does the sled go before
 stopping?
   f k   k FN   k mg       ma
   v v
    2    2
           v v  2    2        a   k g
x      0            0
     2a   2( k g )
         (4.00 m/s)      2
                          16.3 m
    2(0.0500)(9.80 m/s )
                       2
Newton’s Third Law of Motion
 Whenever a body
 exerts a force on a
 second body, the
 second body exerts
 an oppositely
 directed force of
 equal magnitude
 on the first body.
If an astronaut pushes on the   92 kg
spacecraft with a force P = +36
N, then, according to Newton’s
third law of motion, the
spacecraft simultaneously
pushes back on the astronaut                  11 000 kg
with a force –P.

Acceleration of the spacecraft
         P    36 N
    as               0.0033 m/s 2
         ms 11 000 kg

      Acceleration of the astronaut
                    P  36 N
              aA             0.39 m/s 2
                   mA   92 kg
              Second Law: object tends to remain force or in
              Third Law: when one body exerts a force on another
    Newton’s First Law: an a non-zero net externalat reston an object
    produces second body exerts a force equal in magnitude, but
    motion at an acceleration onunless a net proportional to the net
    body, the constant velocity that object force acts on that object
    oppositely directed on the first body
    force and inversely proportional to its mass.




                                FN           F     y    FN  mg  0
                                        F
F
                                                   F     x    F  ma

                                        W = mg
    Newton’s Second Law: a non-zero net external force on an object
              First Law: when one body exerts a at rest or
    Newton’s Third Law:an object tends to remain force on in
    produces body, the second that object proportional to the net
    another an acceleration on body exerts a force acts on
    motion at constant velocity unless a net force equal in that
    force and inversely proportional to its mass.
    magnitude, but oppositely directed on the first body
    object




                                  FN         F   y    FN  mg  0
                             fS          F
F
                                                F     x    F  fS  0
          f

                                       W = mg

				
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posted:9/1/2012
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