Two equal -ve charges -q are fixed at the points (0, a) and (0, -a) on the y-axis. A positive charge Q is released from rest at the point (2a, 0) on the x-axis. The charge will:

Component of force on charge +Q at P, along x-axis.

${\mathrm{F}}_{\mathrm{x}}=\frac{2\mathrm{Qq}}{4{\mathrm{\pi \epsilon }}_0\left({\mathrm{a}}^2+{\mathrm{x}}^2\right)}\cos \mathrm{\theta }$

$=\frac{2\mathrm{Qq}}{4{\mathrm{\pi \epsilon }}_0\left({\mathrm{a}}^2+{\mathrm{x}}^2\right)}\times \frac{\mathrm{x}}{\sqrt{{\mathrm{a}}^2+{\mathrm{x}}^2}}$

$=\frac{2\mathrm{Qqx}}{4{\mathrm{\pi \epsilon }}_0{\left({\mathrm{a}}^2+{\mathrm{x}}^2\right)}^{3/2}}$

Which is not directly proportional to x. So, motion is oscillatory but not SHM