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Homework Help: Electric Potential two metal balls find final velocities

  1. Feb 7, 2012 #1
    1. The problem statement, all variables and given/known data
    Two metal balls of mass m1 = 2 g (diameter = 2 mm) and m2 = 6 g (diameter = 6 mm) have positive charges of q1 = 2 nC and q2 = 7 nC, respectively.

    A force holds them in place so that their centers are separated by 8 mm. What will their velocities be after the force is removed and they are separated by a large distance? (Enter the magnitudes of the velocities.)
    v1 =
    v2 =

    2. The attempt at a solution
    intiial potential energy of the system E= kq1q2/d
    d = .008 m
    k = 9 x 10^9
    .5m1v1^2 + .5m2v2^2 = E
    as there are no external force on the total system momentum is conserved???
    m1v1 = m2v2
  2. jcsd
  3. Feb 8, 2012 #2
    Yes. The momentum of the system is conserved.
    Conservation of energy + conservation of momentum will solve the problem.
    The key is to recognize what happens when the two charges are separated by a large distance ie. distance d between them tends towards infinity.
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