Two fixed very large conducting plates A and B of area S carry charges -Q and q respectively where $Q>q>0$. A third identical plate C carrying a charge Q is released at distance d from B. Third plate collides with B. Assume collision is elastic and time of collision is sufficient to re-distribute charge among B and C (Neglect gravity):

Before collision, electric force 
$F_C=\frac{(Q-q)Q}{2{\in }_{0}S}$ (towards left)
During collision, redistribution of charge takes place between B and C, let charge on them are  $q_{1}and{q}_2$

(Charges on outer most surfaces will be equal)    
$\begin{array}{l}{q}_1=Q+\frac{q}{2}\\ q_2=\frac{q}{2}\end{array}$
Force on plate C after collision
$=\frac{(-Q+Q+\frac{q}{2})}{2{\in }_{0}S}\frac{q}{2}$

$=\frac{{\left(\frac{q}{2}\right)}^2}{2{\in }_{0}S}$ (towards right)

After collision, electric field between plates B and C is zero. So kinetic energy of plate C increases and P.E, between the plates remains constant.