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Homework Help: Potential energy of an object

  1. Dec 7, 2004 #1
    The potential energy of an object is given by U(x)=0.5kx^2+3kx, where U is joules, x is in meters and k is a constant. What is the force acting on the object when x=0?

    PE=Fd
    0.5kx^2+3kx=Fd
    0.5kx+3k=F
    F=3k

    The answer is -3k, why?
     
  2. jcsd
  3. Dec 7, 2004 #2
    Recall Work = -(delta)PE

    Fd = -(0.5kx^2 + 3kx)
    F = -0.5kx - 3k
    F = -3k , x = 0
     
  4. Dec 7, 2004 #3
    what is work for KE?

    W=.5mv^2
    or
    W=-.5mv^2
     
  5. Dec 7, 2004 #4

    Doc Al

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    Staff: Mentor

    The relationship between the potential function and the force is this: [itex]F = - dU/dx[/itex].
     
  6. Dec 7, 2004 #5
    would the object's Kinetic energy be positive then?
     
  7. Dec 7, 2004 #6

    Doc Al

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    Staff: Mentor

    Beats me. That depends on the total mechanical energy, which you have not specified.
     
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