Consider making an analogy between the gravitational potential energy associated with a mass being moved near the surface of the earth and the electric potential energy associated with a moveable charge. electric potential (not electric potential energy) is to charge as is to mass.

Gravitational potential

Explanation:

The gravitational potential at a point in the gravitational field is defined as the work per unit of mass that a force must carry out to transport a body, at a constant speed, from a fixed reference location to that point.

The negative sign indicates that the work done by the gravitational force implies a decrease in gravitational potential.

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.