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Homework Help: Kinetic energy calculation

  1. Jan 3, 2007 #1
    1. The problem statement, all variables and given/known data

    A boy of 60 kgtakes 5 s to run up the slope in the diagram. (Please refer to the attachment) What is his average kinetic energy?

    2. Relevant equations



    3. The attempt at a solution

    I don't know the meaning of 'average' kinetic energy. Is this mean kinetic energy per unit time (per second)? How can I calculate it without knowing the velocity of the boy?
     

    Attached Files:

  2. jcsd
  3. Jan 3, 2007 #2

    Hootenanny

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    Staff Emeritus
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    Gold Member

    His average kinetic energy is the energy corresponding to his average speed
     
  4. Jan 5, 2007 #3
    How can I know the kinetic energy of the boy without knowing the speed?
    My stupid thought was that to use the increase in gravitational potential energy of the boy but I can't get the answer. Why?
     
  5. Jan 5, 2007 #4
    KE=1/2mv^2

    average kintetic energy would come from his average velocity. notice how the distance he went is expressed as a right triangle

    WARNING: ANSWER BELOW
    try not to look at the answer if you can solve it yourself. i made it white so you must high light it and not see it 'on accident'





    [pythagorean theorum tells us the distance he ran was 5 meters and we're given he ran it in 5 seconds, so average velocity is 1 m/s. 1^2 is still 1, and m is 60 so divide it by two and you get 30 N]
     
  6. Jan 5, 2007 #5

    Doc Al

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    Staff: Mentor

    Assume that he runs up at a constant speed. (Since KE is proportional to speed squared, I think this is an ill-posed problem.)
     
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