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Homework Help: Total energy help

  1. Mar 12, 2009 #1
    1. The problem statement, all variables and given/known data

    it is a multi part problem i found most just need help on last two but gave answers to the ones i found because they are needed to determin the answer.

    What is the translational kinetic energy of the real system, at the instant shown in the right diagram?
    Ktrans = 10.412 J

    What is the distance through which the gravitational force acts on the real system? .19 m

    How much work is done on the system by the gravitational force?
    Wgrav = -16.758 J

    What is the distance through which your hand moves? .72 m

    How much work do you do on the real system?
    Whand = 102.96 J

    At the instant shown in the right diagram, what is the total kinetic energy of the real system?
    Ktotal = J

    What is the rotational kinetic energy of the machinery inside the box?
    Krot,f = J

    2. Relevant equations
    from a question earlier on my assingment i found that only
    translational and rotational kenetic energy are pressent
    will need conservation of energy
    delta(E) = W + Q
    i assumed Q = 0 because we have not went in depth about heat trasfer due to friction and all that stuff.

    3. The attempt at a solution
  2. jcsd
  3. Mar 12, 2009 #2
    oops i forgot to show my work ok here is what i did.
    inital state box has no energy it is on the ground and not moving.
    final state it is .19 meters above the ground moving in +y direction and spinning

    i took delta(E) = w + Q Q=0
    so delta(E) = W
    W would equal work done by your hand (102.96) plus work done by gravity (-16.758) since gravity is - it is actually suptracting so i get delta(E) = 86.202
    that sould be total energy but we only want kenetic energy so i found potential energy using mgh (mass was given as 9 kg) so 9*9.8*.19 = 16.758. that should be potential so
    i subtracted that from total 86.202 - 16.758 = 69.444
    that is my answer but it says im wrong. any help would be great!!
    thanks and sorry about the lenght of this post.
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