The ball gets positively charged when touches the bottom plate and will experience a constant force due to electric field generated due to the potential difference between the plates which is directed upwards. The balls get accelerated upwards under the influence of this force.

After hitting the top plate, the balls will get negatively charged and will now get attracted to the bottom plate which is positively charged. The motion of the balls will be periodic but not $SHM$.

$2.$ $I_{a v} \propto \frac{Q}{t}$

$\text { Here } Q \propto V _0$

$\text { Also } \quad S = ut +\frac{1}{2} at ^2$

$h=\frac{1}{2} \frac{ QE }{ m } t ^2=\frac{1}{2}\left(\frac{ Q \times 2 V_0}{ mh }\right) \times t ^2$

$\therefore \quad t \propto \frac{1}{V_0} \quad-\text { (iii) } \quad\left[\because Q \propto V _0\right]$

$I _{ av } \propto \frac{ V _0}{1 / V _0}= I _{ av } \propto V _0^2$