Docstoc

Work _ Energy

Document Sample
Work _ Energy Powered By Docstoc
					Work & Energy



  By Christos
                         Work
   Work is defined as a force acting upon an
    object to cause a displacement.
  Be aware: Work  Force  Distance is not always true
 If the acting force has no component in the direction of
  the movement (or if the force is acting perpendicular at the
  direction of the movement) then the force does not cause
  the movement and therefore it produces 0 work.
 No matter how big force one exerts to an object, if there is
  no resulting displacement the work produced will be zero.
 In the special case, however, in which a force is acting in
  the direction of the displacement, then and only then, one
  can claim that:      Work  Force  Distance
           Work done by a Force
   Work done by a force=F x distance moved in the direction of force
    Be Extra Careful: Work is not always Force x Distance:
   If the acting force has no component in the direction of the movement
    (or if the force is acting perpendicular at the direction of the
    movement) then the force does not cause the movement and therefore
    it produces 0 work.
    No matter how big is the force acting upon an object if there is no
    displacement there is no work.
   However, in the case that an force is acting in the same direction of the
    displacement, then and only then, one can claim that: Work=Force x
    displacement.
Lets see if we got it right!
    The direction of
    the force is in the
                                                        Force x distance
    same direction        The force pushing a car
    that the object       along a road
    moves

    The direction of
    the force is in the                                 -Force x distance
    opposite              The force the brakes exert
    direction that the    to stop a car.
    object moves

    The direction of
    the force is
                                                               0
    perpendicular to      The gravitational force the
    the direction that    Earth exerts on the Moon
    the object moves

    The object                                                 0
                          The force you exert when
    doesn't move
                          pushing on a wall
         The truth of the matter
   Work  Force  Distance  cos




a force will do work only if the force has a component in
            the direction that the object moves
Special cases we’ve seen so
            far
                          Work = Force x Distance
                                  (cos00=1)

                          Work = - Force x Distance
                               (cos1800=-1)

                         Work = 0 since cos900=0

   If displacement is equal to zero then the
       work done is zero no matter the
    direction and magnitude of the Force .
   NO DISPLACEMENT = NO WORK
             Kinetic Energy
   Kinetic energy is the energy of
    motion. Kinetic Energy  1 2  m  v 2
              Where :
              m  mass of the object
              v  speed of the object
 To derive this equation one should
  use advance maths!(so forget it)
 However, you should understand
  what this equation means …..>
… Kinetic Energy Continued
    Kinetic Energy  1 2  m  v                              2


 The kinetic Energy of an object is directly proportional to
  its mass.
 The Kinetic Energy of an object is directly proportional to
  the square of its speed. That means that for a twofold
  increase in speed, the kinetic energy will increase by a
  factor of four; for a threefold increase in speed, the kinetic
  energy will increase by a factor of nine
 Kinetic Energy is a Scalar quantity.
 Units of Kinetic Energy Kg m2/s2 = 1 Joule
      Lets check our understanding
Suppose that you were
  captured by an evil
  physicist who gave you
  the following choice:
You must either:
    Stand in front of a
     1000 kg. truck moving
     at 1 m/s, or
    Stand in front of a 1
     kg. meatball moving at
     1000 m/s.
               Potential Energy
Potential energy is the stored   – Gravitational
energy of position possessed       potential energy is
        by an object.              the energy stored in
                                   an object as the result
                                   of its vertical
                                   position
                                 – Elastic potential
                                   energy is the energy
                                   stored in elastic
                                   materials as the result
                                   of their stretching or
                                   compressing
   … Potential Energy Continued
PE grav  Weight  Height  mass  accelerati on due to gravity  Height
 PE grav  m  g  h Units: Kg  m s 2  m  Kg  m 2 / s 2  Joule
            Mechanical Energy
   Mechanical energy is the total energy which is possessed
    by an object due to its motion and/or its stored energy of
    position.
       … Mechanical Energy
           Continued
 The total amount of mechanical energy is merely
  the sum of the potential energy and the kinetic
  energy. This sum is simply referred to as the total
  mechanical energy :
        TME=Kinetic Energy+Potential Energy
             (units make a wild guess!)
 an object with mechanical energy is able to do
  work on another object.
       Work Energy Theorem
   Plethora of ways to categorize forces.
    – contact forces or as action-at-a-
      distance forces
    – External-Internal Forces:
          External forces can change the total mechanical
           energy while doing work on a system
          Internal forces can not change the total mechanical
           energy of a system while performing work upon it.
                External Forces
   applied forces,
    normal forces,
    tensional forces,
    friction forces,
    and air resistance
    forces.
   Work=change of
    mechanical energy
TMEBefore  Work  TME After 
KE i  PE i  W  KE f  PE f
          Internal Forces
 Internal forces include gravitational
  forces, magnetic forces, electrical
  forces, and spring forces.
 When work is done upon an object by
  an internal force the total mechanical
  energy (KE + PE) of that object remains
  constant. In such cases, the object's
  energy changes form.
           The roller coaster +Book
http://www.physicsclassroom.com/mmedia/qt/energy/coaster.mov

http://www.batesville.k12.in.us/Physics/PhyNet/Mechanics/Energy/lifting_a_book
.htm

http://www.batesville.k12.in.us/Physics/PhyNet/Mechanics/Energy/pushing_a_bo
ok.htm

http://www.batesville.k12.in.us/Physics/PhyNet/Mechanics/Energy/Work_as_Area
.html
Check our Understanding
                                         KE to PE or PE to KE?
   Description of Motion                 Explain.




 1 A ball falls from a height of 2
   meters in the absence of air
   resistance.
 2 A skier glides from location A to
   location B across the friction free
   ice.
 3 A baseball is travelling upward
   towards a man in the bleachers.

   A bungee chord begins to exert
   an upward force upon a falling
   bungee jumper.

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:7
posted:12/20/2011
language:
pages:18