The magnitude of the kinetic frictional force between the surface and the 5kg block is 8n. the coefficient of kinetic friction between the 2kg block and the surface is 0,31 a. calculate the magnitude of the kinetic frictional force between 2kg block and the surface
b. calculate the magnitude of the tension in the string connecting the two blocks

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.

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.