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Potential Energy and Bernoulli

  1. Nov 14, 2004 #1
    [tex]P_1 + \frac{1}{2} \rho (v_1)^2 + \rho g h_1 = P_2 + \frac{1}{2} \rho (v_2)^2 + \rho g h_2 [/tex]

    In this equation (and regular energy equations for that matter) is g= 9.8 or -9.8 m/s^2 ?

    To make sense mathematically I believe it has to be 9.8 or else pressure and velocity would increase as a fluid increases its height. I think my textbook needs to define when g is negative and when it is positive can get confusing sometimes. In kinematic equations you can pick a reference frame and set it positive or negative yourself but in energy equations it can get confusing.
    For example [tex] W_{gravity} = - \Delta PE [/tex] and also [tex] \Delta PE = mg \Delta y [/tex] . If it weren't for my teacher showing me that work was the magnitude of F X magnitude of distance X cosine of lesser included angle (which my book neglects to mention) I'd be completely confused.
  2. jcsd
  3. Nov 14, 2004 #2

    Doc Al

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    Staff: Mentor

    In common usage g is always the magnitude of the acceleration due to gravity; it is a positive quantity (e.g., 9.8 m/s^2). Thus the acceleration due to gravity is g, downward.
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