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Homework Help: Electric potential of tiny spheres question

  1. Feb 2, 2013 #1
    1. The problem statement, all variables and given/known data
    Two tiny metal spheres A and B of mass mA = 3.40 g and mB = 3.90 g have equal positive charges q = 4.50 µC. The spheres are connected by a massless nonconducting string of length d = 1.00 m, a distance that is much greater than the radii of the spheres.
    (a) What is the electric potential energy of the system?
    (b) Suppose you cut the string. At that instant, what is the magnitude of the acceleration of each sphere?
    __m/s2 (Sphere A)
    __m/s2 (Sphere B)
    (c) A long time after you cut the string, what is the speed of each sphere?
    __m/s (Sphere A)
    __m/s (Sphere B)

    2. Relevant equations


    3. The attempt at a solution

    My teacher told me to use conservation of energy and momentum.

    a) U=kq^2/r
    U=((9*10^9)(4.50 µ)^2/1
    U=.18225 J

    i dont understand part B-C
  2. jcsd
  3. Feb 3, 2013 #2


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    Homework Helper

    For b), find the force acting on both particles and use Newton's First Law to find the accelerations.

    For c), use conservation of momentum and energy. What is the potential energy when the particles are very far away? How are the velocities related?

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