A sphere carrying a charge +Q is fixed. A small sphere carrying a charge +q is placed near the bigger sphere and released from the rest. Due to repulsion, the smaller sphere will move away from the bigger sphere with

The acceleration of the smaller sphere is
$a=\frac{F}{m}=\frac{1}{4\pi {\in }_0}\frac{Qq}{r^{2}m}$
As the smaller sphere moves away, r increases. Hence a decreases. Since a is always in the direction of motion, the velocity of the smaller sphere is always increase.