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Which formula?

  1. Oct 20, 2004 #1
    A 16 kg object is acted on by a conservative force given by F = (-2.9)x + (-4.8)x2, with F in newtons and x in meters. Take the potential energy associated with the force to be zero when the object is at x = 0. What is the potential energy of the system associated with the force when the object is at x = 2.0 m?
    I found this answer to be 18.6 J

    But I can't get the second half of the question...

    If the object has a velocity of 5.0 m/s in the negative direction of the x-axis when it is at x = 5.0 m, what is its speed when it passes through the origin?

    I thought I could do it using the formulas for constant acceleration but I don't know the acceleration or the time. Any suggestion for what formula to use?
  2. jcsd
  3. Oct 20, 2004 #2
    Use the princlipple of conservation of energy. You have potential energies for both locations and a kinetic energy ao one location. The problem states the force acting on the particle is conservative so POCOE is fairly easy to implement here. Just realize that the sum of the potential and kinetic energy at 5m equals the sum of the energys at 0m.

    Good luck.

    [edit]Sorry, you need to use work-energy here. The force applied over a distance is work which must be factored in. Basically, PE1+KE1+W=PE2+KE2.
    Last edited: Oct 20, 2004
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