A parallel plate capacitor whose capacitance C is 14 pF is charged by a battery to a potential difference $\mathrm{V}=12 \mathrm{~V}$ between its plates. The charging battery is now disconnected and a porcelin plate with $\mathrm{k}=7$ is inserted between the plates, then the plate would oscillate back and forth between the plates with a constant mechanical energy of $\_\_\_\_$ pJ.