Answer
Slight changes in position of the ball disrupt the equilibrium.
The equilibrium is unstable.
Work Step by Step
The magnitude of the electrostatic force is given by 19-5,
$F =k\displaystyle \frac{|q_{1}||q_{2}|}{r^{2}}$
(Force is inversely proportional to the square of the distance between the charges.)
If we displace the ball slightly downward, the distance will increase slightly, and as a consequence, the electrostatic force will decrease. The weight is the same as before, and now it is greater than the electrostatic force, so a net force will exist, pointing down, causing the -q charge to accelerate.
If we displace the ball slightly upward, the distance will decrease slightly, and as a consequence, the electrostatic force will increase. The weight is the same as before, so a net force will exist, pointing up, causing the -q charge to accelerate.
Slight changes in the position of the ball disrupt the equilibrium. The equilibrium is unstable.
