You serve a volleyball with a mass of 2.1 kg. the ball leaves your hand with a speed of 30 m/s.

You haven't asked any question about this situation, so we have to guess.  Perhaps the question is: Why are you playing the game with a ball that weighs more than 4.6 pounds, and how can you protect your hand when it's your serve ?

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.