A2-kg objects is moving horizontally with a speed of 4 m./s. how much net force is required to keep the object moving with the same speed and in the same directions

Look at Newton's Second Law of Motion.  F = m A

No, I mean REALLY LOOK AT IT.

The FORCE you need to give something an acceleration of A is          (MASS) x (A).

If your object is moving with a steady speed in a straight line, then it has no acceleration.  A = Zero.

The force it takes to keep it moving that way is (mass) x (zero) = Zero .

An object that's moving in a straight line at a steady speed doesn't need ANY force to keep going.

Force does not CAUSE constant uniform motion. Force CHANGES motion.

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.