Two point charges q_1 = 2.10 nc and q_2 = -6.20 nc are 0.100 m apart. point a is midway between them; point b is 0.080 m from q_1 and 0.060 m from q_2 (the figure (figure 1) ). take the electric potential to be zero at infinity.

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.