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Workdone and Energy

  1. Sep 24, 2015 #1
    Screen Shot 2015-09-25 at 1.46.45 AM.png
    Answer stated is D

    Screen Shot 2015-09-25 at 1.46.28 AM.png
    Answer stated is D



    My attempt at the solution:


    For the first question,
    the calculations are as follow:

    A. P=I2R = 42×12 = 192 J/s -- Missing information
    B. Energy = 1/2 mv2 = 15×9= 135J
    C. Energy = mgh = 10×10×1 = 100J
    D. Work done = 0J ; moving against zero friction.

    My answer will therefore B for the first question.

    For the second question,
    Work done = Ffric × d = 70N×4m = 280J

    The information with regards to the mass of the object is irrelevant since the friction coefficient is not stated and the frictional force has been stated in the problem.

    My answer is A for the second question.
     
  2. jcsd
  3. Sep 24, 2015 #2

    TSny

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    I like your answers. It appears that the wrong answer D in each case arises from the mistaken view that work must be done against the force of gravity and that this work is equal to the weight times the distance moved (even though the weight force is perpendicular to the displacement).

    The wording of the questions is not very good. In option D for #14 and in #25, there is no information about whether or not the speed of the object remains constant . In #25 the phrase "energy exerted" is a bit odd. We don't normally think of "exerting" energy.
     
  4. Sep 24, 2015 #3
    Thanks for your prompt reply and assurance that my conceptual knowledge is not at risk. May also I get you to acknowledge if the information provided for #14 (A) is incomplete to deduce the energy consumed.
     
  5. Sep 24, 2015 #4

    TSny

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    You are completely right. You would also need to be given a time period in order to get energy in #14 (A).
     
  6. Sep 24, 2015 #5

    CWatters

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    I agree. As written the answers are B and A. Very badly worded questions.
     
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