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Defining forces on system help

  1. Feb 26, 2009 #1
    i was reviewing for my exam and this was one of the review questions. They dont give us the answers (dont know why it is pointless studying the wrong stuff) so i was wondering if anyone knew the right answers. each system can have more than one answer and here is the problem word for word

    Starting from rest, a woman lifts a barbell with a constant force F through a distance h, at which point she is still lifting, and the barbell has acquired a speed v. Let Ewoman stand for the following energy terms associated with the woman:

    Ewoman = Echemical,woman + Kwoman (moving arms etc.) + Ugrav,woman+Earth + Ethermal,woman
    The change in the kinetic energy of the barbell is (1/2)mv2 - 0 = (1/2)mv2.
    The general statement of the energy principle is Esys = Wext. We'll consider terms on the left side of the equation (the Esys side, changes in the energy inside the system) and terms on the right side (the Wext side, energy inputs from the surroundings).

    I. System: Woman + barbell + Earth
    For the system consisting of the woman, the barbell, and the Earth, which of the following terms belong on the left side of the energy equation (the Esys side)?

    (1/2)mv^2
    +mgh
    -mgh
    Fh
    none of these terms (left side is 0)
    (1/2)mv2
    -Fh
    Ewoman





    For the system consisting of the woman, the barbell, and the Earth, which of the following terms belong on the right side of the energy equation (the Wext side)?

    2-(1/2)mv2
    -mgh
    none of these terms (right side is 0)
    Fh
    Ewoman
    +mgh
    (1/2)mv2
    -Fh




    II. System: barbell only
    For the system consisting of the barbell only, which of the following terms belong on the left side of the energy equation (the Esys side)?

    3-(1/2)mv2
    -Fh
    +mgh
    (1/2)mv2
    Ewoman
    Fh
    none of these terms (left side is 0)
    -mgh




    For the system consisting of the barbell only, which of the following terms belong on the right side of the energy equation (the Wext side)?

    4Fh
    none of these terms (right side is 0)
    +mgh
    Ewoman
    -Fh
    -mgh
    (1/2)mv2
    -(1/2)mv2




    III. System: barbell + Earth
    For the system consisting of the barbell and the Earth, which of the following terms belong on the left side of the energy equation (the Esys side)?

    5Ewoman
    +mgh
    -(1/2)mv2
    Fh
    -Fh
    (1/2)mv2
    none of these terms (left side is 0)
    -mgh




    For the system consisting of the barbell and the Earth, which of the following terms belong on the right side of the energy equation (the Wext side)?

    6none of these terms (right side is 0)
    Fh
    (1/2)mv2
    +mgh
    Ewoman
    -Fh
    -mgh
    -(1/2)mv2
     
  2. jcsd
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