The nucleus is always positive since thats where the protons are located and the neutrons have no charge....
The total amount of energy always stays the same because of the law of conservation of energy, meaning that there is always the same amount of energy, it just gets turned into different forms like potential energy, kinetic energy, sound, thermal, etc.
Work done is given by product of force and displacement due to that force
So here we will have
here we know that
Now work done is given as
so it will do 16 J work to move the box