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Homework Help: The slope of a kinetic energy vs. potential energy graph

  1. Apr 13, 2012 #1
    1. The problem statement, all variables and given/known data

    I have plotted a graph of Gravitational potential energy (y-axis) vs. kinetic energy (x-axis), and I have found the value of the slope to equal -1.4. The thing is I dont know what the -1.4 equals to? Since I have to find the percentage error, I need to know the theorectical value of the experiment, to put into the equation of % error. I have tried several ways to determine what the value would be like but I don't think they make sense:

    3. The attempt at a solution

    (kg(m/s^2)m)/((kg(m^2/s^2))/2) which equals finally 2 (theo value)

    and when I put into percentage error formula:

    (|theo-exp|/|theo|)x100→(2-(-1.4)/2)x100→1.7x100=170% (which is impossible)

    or something I found on the internet:

    For a freely falling body,
    P.E. + K.E. = cosntant
    => x + y = constant, where x = P.E. and y = K.E.
    => P.E. vs. K.E. graph is a line with slope = - 1

    I don't understand how -1 should be the theo value of the slope... But it works perfectly in my equation for percentage error!

    (|theo-exp|/|theo|)x100→(-1-(-1.4)/-1)x100→0.4x100=40% (which makes more sense)

    Could someone help me understand this? And tell me my mistakes? It doesn't make any sense to me! Thanks!

    2. Relevant equations

  2. jcsd
  3. Apr 13, 2012 #2

    Write the equation as

    PE + KE = C, where C is a constant.

    Then put it in the form of y = mx + b

    What is m?
  4. Apr 14, 2012 #3
    Uh well "m" equals -1 right?
  5. Apr 15, 2012 #4
    That's correct.
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