A sphere with radius 10 cm is filled with a uniform charge distribution. The magnitude of the electric field at a point 5 cm from the center of the sphere is 3014 N/coul. Use this fact to calculate the charge density ρ inside the sphere.

atomic = no of protons.

mass number-atomic number = neutron number

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.