Work, Energy, and Power by VRX8nK

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									Work, Energy, and
     Power
   A.   Work
   B.   Work: Practice Problems
   C.   Potential Energy
   D.   Potential Energy: Springs
   E.   Kinetic Energy
   F.   Mechanical Energy
   G.   Power
A. Work
   A force acting upon an object to cause a
    displacement


   F = force
   d = displacement
   theta is the angle between the force and
    the displacement vector
A. Work
A. Work
   A force acts upward upon an object as it is
    displaced rightward
   The force vector and the displacement
    vector are at right angles to each other
   The angle between F and d is 90 degrees
   Calculate Work
   cos 90 = 0
   A vertical force can never
    cause a horizontal displacement
A. Work
   Determining the angle
   It is the angle between the force
    and the displacement vector
   A force is applied to a cart to pull it up an incline
    at constant speed
   Several incline angles were used; yet, the force
    was always applied parallel to the incline
   The displacement of the cart was also parallel to
    the incline
   Since F and d are in the same direction, the angle
    was 0 degrees
A. Work
   Measured in Joules (J)
   The Joule is the unit of work.
    1 Joule = 1 Newton * 1 meter
    1J = 1 N * m

   One Joule is equivalent to one Newton of
    force causing a displacement of one meter
B. Work: Practice Problems
B. Work: Practice Problems

   A = 500 J

   B = 433 J

   C = 750 J
B. Work: Practice Problems
   A 10-N forces is applied to push a block
    across a friction free surface for a
    displacement of 5.0 m to the right
   Calculate Work.

   W = 50 J
B. Work: Practice Problems
   A 10-N frictional force slows a moving block
    to a stop after a displacement of 5.0 m to
    the right
   Calculate work.

   W = -50 J
C. Potential Energy
       the stored energy of position possessed by an
        object


   2 Types
       Gravitational PE
       Elastic PE
C. Potential Energy

   Gravitational PE
       the energy stored in an object as the result of its
        vertical position (height)
       dependent on two variables
            mass of the object
            height
C. Potential Energy

   Elastic PE
       the energy stored in elastic materials as the
        result of their stretching or compressing
       Stored in rubber bands, bungee chords,
        trampolines, springs, an arrow drawn into a bow
       the more stretch, the more stored energy
C. Potential Energy

   Elastic PE
       the amount of force is directly proportional to the
        amount of stretch or compression (x)
       the constant of proportionality is known as the
        spring constant (k).
C. Potential Energy

   Elastic PE
D. Potential Energy: Springs
   Hooke’s Law
       If a spring is not stretched or compressed, then
        there is no elastic potential energy stored in it
       The spring is said to be at its equilibrium position
            The equilibrium position is the position that the
             spring naturally assumes when there is no force
             applied to it
E. Kinetic Energy
   The energy of motion
   An object which has motion - whether it be
    vertical or horizontal motion - has kinetic
    energy
E. Kinetic Energy
   Types of KE:
   vibrational (the energy due to vibrational
    motion)
   rotational (the energy due to rotational
    motion)
   translational (the energy due to motion from
    one location to another)
E. Kinetic Energy
   Translational KE
   Depends upon two variables
       the mass (m) of the object
       the speed (v) of the object




            m = mass of object
            v = speed of object
E. Kinetic Energy
   Kinetic energy is a scalar quantity; it does
    not have a direction
   The kinetic energy of an object is
    completely described by magnitude alone
   KE is measured in Joules (J)
F. Mechanical Energy
   The energy which is possessed by an
    object due to its motion or its stored energy
    of position
   Can be PE or KE
   An object which
    possesses mechanical
    energy is able to do
    work
F. Mechanical Energy
F. Mechanical Energy
   The mechanical energy of an object can be
    the result of its motion and/or the result of
    its stored energy of position
   The total amount of mechanical energy is
    merely the sum of the potential energy and
    the kinetic energy
   TME = PE + KE
   TME = PEgrav + PEspring + KE
G. Power
   The rate at which work is done
   The work/time ratio
   The standard metric
    unit of power is the Watt
   Watt is equivalent to a Joule/second
G. Power
   Most machines are designed and built to do work
    on objects
   All machines are typically described by a power
    rating
   The power rating indicates the rate at which that
    machine can do work upon other objects
   The power of a machine is the work/time ratio for
    that particular machine
G. Power
G. Power
   Suppose that Ben elevates his 80-kg body
    up the 2.0 meter stairwell in 1.8 seconds. If
    this were the case, then calculate Ben's
    power rating
G. Power
   Suppose that Ben elevates his 80-kg body
    up the 2.0 meter stairwell in 1.8 seconds. If
    this were the case, then calculate Ben's
    power rating

								
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