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Dynamics_

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									Dynamics
 Kinematics: the study of HOW objects
  move
 Dynamics: the study of WHY objects
  move the way they do
    ◦ Force any push or pull on an object
      Vector quantity
      Unit: Newtons (N)    1 N = 1 kgm/s2
    ◦ 2 types of Forces
      Contact Force: involves contact with the object
      Field Force: force act on the object over a
       distance…no contact
    FORCES
 Contact Forces                  Field Force
 Applied Force: (Fa): the        Force due to Gravity:
  force a person exerts on an
  object                           (Fw): the measure of attraction
                                     between an object and the center
 Frictional Force: (Ff): the
                                     of the earth; the object’s
  force that opposes motion.
                                     WEIGHT. Always downwards.
  Always between two
  surfaces. Acts opposite the        FREE BODY DIAGRAMS:
  direction of an object’s           show all the forces acting on an
  motion                             object; help determine how an
                                     object will move.
 Normal Force: (FN):
  support force. Always
  perpendicular to the surface
Newton’s 1st Law of Motion
   The Law of INERTIA
    ◦ Inertia: the property of an object to resist
      any change in its motion
    ◦ This means…an object at rest will stay at rest;
      an object in motion will stay in motion in a
      STRAIGHT LINE unless acted upon by an
      outside force.
    ◦ Simply put…objects like to keep doing what
      they’re doing.
     Mass and Inertia
   Mass and inertia are
    DIRECTLY related.
    ◦ More mass means
      more inertia.
    ◦ More inertia means
      it’s HARDER to
      change the object’s
      motion.
        Net Force
 To find net force (Fnet) or
  resultant force, subtract
  forces in opposite direction.
 Fnetx = (forces to right) –
  (forces to the left)

   Fnety = (forces up) – (forces
    down)
       Equilibrium
   Equilibrium: forces are
    EQUAL and OPPOSITE
    ◦ Fnet = 0 N
    ◦ The object could be:
       At rest (v = 0 & a = 0)
       Moving with CONSTANT
        velocity
   The object is not
    ACCELERATING when it
    is in equilibrium. (But, it
    can be moving.)
        Weight vs. Mass
   Weight (Fw) is a measure of the force due to
    gravity pulling an object downwards; changes
    depending on how much gravity there is.
    ◦   Unit: Newtons (N)
    ◦   Weight = mass x gravity
    ◦   Fw = mg
    ◦   Gravity on earth (g) = 9.81 m/s2
   Mass is how much matter an object is made
    up of; it stays the same no matter where in
    the universe you are
    ◦ Unit: kg
Newton’s 2nd Law of Motion
 A net force causes an object to accelerate
 Acceleration is directly proportional to
  the net force on the object
    ◦ The greater the force, the faster the object
      will accelerate
   Acceleration is inversely proportional to
    the mass of the object
    ◦ The larger the mass, the slower it will
      accelerate
 Fnet    = ma
Newton’s 3rd Law of Motion
   For every action, there is an equal but
    opposite reaction.
    ◦ Forces always occur in pairs.
      Action Force
      Reaction Force

     Force on                          Force on
     object A                          object B
    caused by                         caused by
     object B                          object A
                  A        B
Friction
   Any force that resists the motion of an
    object
    ◦ Acts parallel to the surfaces in contact.
    ◦ Acts in the opposite direction of the object’s
      motion.
   Amount of Ff depends on:
    ◦ The type of materials in contact
    ◦ The amount of force pressing the surfaces
      together
2 Types of Friction
   Static Friction
    ◦ The friction keeping objects at rest
   Kinetic Friction
    ◦ The friction slowing down moving objects
**Static friction is always greater than Kinetic friction
  because it is easier to keep an object moving than it
  is to start is motion. (Think INERTIA!)
**Coefficient of Friction (μ): number describing the
  type of material. The higher the number, the more
  friction produced. No UNITS!
Falling Objects
 Objects fall at the same rate if they are
  falling in a vacuum (no air).
 Free Fall: Objects fall due to the influence
  of gravity
    ◦ Forces Present:
      Force due to gravity (weight)
      Force of air resistance (friction)
    ◦ Terminal Velocity: the fastest an object can fall
      based on its surface area.
      Fnet = zero
    Terminal Velocity
   As an object falls, it speeds
    up due to gravity. As the
    velocity increases, the
    amount of air hitting the
    object increases, so friction
    increases. When the friction
    is equal to the weight, the
    net force on the object is
    zero. No net force means
    no acceleration.
    CONSTANT VELOCITY!

								
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