Potential Energy of two spheres

[lcam2]

Homework Statement

Two small metal spheres, A and B, of mass mA = 5.5 g and mB = 9.4 g have equal positive charges of 8 μC. The spheres are connected by a non-conducting string of negligible mass and with length d = 1 m, which is much greater than the radii of the spheres.
a) What is the electrical potential energy of the system?
b) Suppose you cut the string. At that instant, what is the magnitude of the acceleration of each sphere?
c) After a very long time, what is the magnitude of the velocity of each sphere?

Homework Equations

U=Electrical potential Energy
U_System = K Q₁Q₂/R

U =-W = -Fd; W= work

F=ma

P= Momentum; P=ma
P_i = p_f

The Attempt at a Solution

a) it is just the electrical potential energy of system.
U_System = K Q₁Q₂/R

b) U=-Fd
Using electric potential energy found in part a, find the force F.
Then F=ma, solve for masses of a and b, respectively.

c) momentum initial= final momentum, inital momentum of both spheres is zero, given that they were released from rest, when connected by the string.

m_a*v_a = -m_b*v_b

I need another equation, to solve the problem
But i don't know what equation to use.
Thanks in advance.

 

[rl.bhat]

Use conservation of energy. At infinity potential energy is zero.