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Homework Help: Work, energy and power

  1. Aug 29, 2010 #1
    A constant force of 9 kN parallel to an inclined plane, moves a body of weight 20 kN through a distance of 40m along a plane at a constant speed. The body gains 12 m height..

    HOW MUCH WORK IS DISSIPATED AS HEAT?

    A/ 120 KJ

    B/ 240 KJ

    C/ 360 KJ

    D/ 600 KJ

    So workdone = f x d

    but i am a bit confuse as the question say the heat dissipated

    so i think that the work done = 20 kN X 12 = 240 KN (heat dissipated) answer

    However the work done is 9 KN X 40 = 360 KJ


    AM I ON THE RIGHT TRACK ????? HELP BY THE WAY ACCORDING TO ME THE ANSWER IS B
     
  2. jcsd
  3. Aug 29, 2010 #2

    Doc Al

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    You know the total work done by the force. Where does that energy go? Some goes to heat, but where does the rest go?
     
  4. Aug 29, 2010 #3
    So some converts to kinetic energy and potential enegy..please just tell me whether i am on the right track and if the answer is correct... thanks in advance
     
  5. Aug 29, 2010 #4

    Doc Al

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    Good. In this case, how much energy goes to KE? How much to PE?

    No, your answer is not correct. But you'll get there.
     
  6. Sep 9, 2010 #5
    So p.e is 20 kN x 12
    What about the kinetic energy..?? no speed...what should i do...
     
  7. Sep 9, 2010 #6

    Doc Al

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    Good!
    You are told that the speed is constant. So how much of the total work goes into KE?
     
  8. Sep 10, 2010 #7

    So i think that its the work done of moving the object to the top this is equal to the kinetic energy..!!! i am right..???
     
  9. Sep 10, 2010 #8

    Doc Al

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    No. Since the speed of the object--and thus its kinetic energy--doesn't change, none of the work is transformed into kinetic energy. There is no change in kinetic energy.
     
  10. Sep 10, 2010 #9
    Awww..but what is the answer..?????..
     
  11. Sep 10, 2010 #10

    Doc Al

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    See my hint in post #2. You know the total work done. And you know how much went to PE and KE. What's left?
     
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