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Slope of an Energy with Friction Graph?

  1. Jan 19, 2010 #1
    1. The problem statement, all variables and given/known data
    An object is pulled at a constant F. KE0+PE0=0, so W=PE+KE. If there were no friction, the slope of a graph (KE+PE=Y-axis, W=X-axis) would be 1 and the y-intercept would be 0. What would the addition of friction do to the slope and y intercept (would the y intercept be more, less, or equal to 0 and the slope more, less, or equal to 1)


    2. Relevant equations

    KE0+PE0+W=PE+KE

    3. The attempt at a solution
    At first I thought that the slope would be less than 1 and the y intercept below 0, since my experimental results seemed to confirm this and since friction would be "taking" energy away. However, one of my classmates told me there should be no change. Can anyone explain this problem to me?
     
  2. jcsd
  3. Jan 19, 2010 #2

    Matterwave

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    It depends on how you define your work. If your work includes the work done by friction, then there would be essentially no change. If your work only includes work done by the original force, there should be a change.
     
  4. Jan 19, 2010 #3
    Just so I can understand this--is this because friction would be taking away from both sides of the equation? Why "essentially" no change?
     
  5. Jan 24, 2010 #4

    Matterwave

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    You have work done by both forces. Usually, we define the work done by your force to be positive. In that case, the work done by friction is negative (and is equal to the loss in KE and PE).

    So, if you include friction, the graph wouldn't change. The PE+KE of the particle would go down because friction is converting that energy into heat, BUT, the negative work of friction would also bring your W down by the exact same amount. If you don't include friction, the graph will change. The work done by your force is the same, but the PE+KE of the particle will go down due to friction.
     
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