We use Coulomb’s Law and Newton’s Second Law to determine the acceleration of charge q.
• The force on q due to Q is
$\mathit{F}=\frac{\mathit{k}\mathit{Q}\mathit{q}}{{\mathit{r}}^2}$
• Acceleration is given by:
$\mathit{a}=\frac{\mathit{k}\mathit{Q}\mathit{q}}{\mathit{m}{\mathit{r}}^2}$
This means that acceleration varies inversely with ${\mathit{r}}^2$. The correct graph should show a decreasing function of r with a sharp drop.