A ball with a net charge of $+450 \mathrm{nC}$ and mass 0.75 g is suspended from a thread of length $12 \mathrm{cm}$ in a uniform electric field of $8.0 \mathrm{kV} / \mathrm{m}$ downward. Do not ignore gravity. The system acts like a pendulum but with a downward electric force added to the gravitational force.
What is the period of oscillation for small amplitudes?