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Potential energy negative, force & displacement co-linear?

  1. Nov 9, 2015 #1
    1. The problem statement, all variables and given/known data
    In the following question, in the solution, why is it that when they calculate the gravitational potential energy at point 2 (impact with spring), it is negative? I know that if both the force and displacement act in the same direction, then the work done should be positive. Why is it not the same thing here?


    XNuSEne.jpg
     
  2. jcsd
  3. Nov 9, 2015 #2
    I think they mean both answers could work when the equation is solved...however one answer would obviously make more sense in this case.
     
  4. Nov 9, 2015 #3
    Oh sorry about that being un-clear. I mean for $$V_g = mgh_2 = (50)(9.81)(-x \sin 20)$$

    why is it negative?
     
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