The electric potential difference of a 125 F capacitor is measured across the terminals of the capacitor and found to be 9.0 V. The potential energy of the capacitor, rounded to two significant figures, is

E=1/2CV^2

where E is potential energy, C is capacitance and V is the voltage.

E=1/2×125×9^2

E=1/2×125×81

E=5062.5

E=5100J to 2 significant figures.

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.