A negative test charge −q is released from rest at a distance r away from a sphere of charge +Q, and a distance 2 r away from a sphere of charge +2Q. The charged spheres are fixed in place. System of charges arranged in a horizontal line. Test charge minus Q is in the middle position. Charge plus Q located a distance R to the left. Charge +2Q located a distance 2R to the right. In what direction does the test charge move immediately after being released?

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.

vapour or gas assuming atmospheriv pressure

it is a negative acceleration