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Homework Help: Particle Kinetic energy problem

  1. Feb 13, 2006 #1
    Particles A (of mass m and charge Q) and B (of mass m and charge 5Q) are released from rest with the distance between them equal to 0.9773 m. If Q = 8 [tex] \mu C [/tex], what is the kinetic energy of particle B at the instant when the particles are 2.9773 m apart? Answer in units of J.
    I really don't know how to do this problem. I tried to set it up by using conservation of energy.
    KE_A + KE_B + U_A + U_B = KE_A + KE_B + U_A + U_B
    qkQ/.9773 + qk5Q/.9773 = KE_A + KE_B + qkQ/2.9773 + qk5Q/2.9773.
    I really don't think this setup is right, and I have no idea how I would be able to solve for the final kinetic energy of B when I don't know the final velocity of A.
  2. jcsd
  3. Feb 14, 2006 #2
    1)Start out with determining what force is acting between those particles. Also, make sure that you have the correct direction of the force vector with respect to an X and Y axis.

    2)Apply Newton's second Law in both the X and Y direction.

    3) Solve the equations to get the particles' velocity and positions as a function of time

  4. Feb 14, 2006 #3
    Since the particles both have a positive charge, I know they will repel. Aside from that force, the normal force and gravity, those are the only forces I know of. Is that right?
  5. Feb 14, 2006 #4
    Coulombic force and gravity, yes.

    Though in this problem, i think you can omit gravity.

  6. Feb 14, 2006 #5
    Wouldn't the columbic force just be in the x-dir? So it would be like
    qE_b- qE_a= ma ?
    Sorry... I'm still confused.
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