At the peak of its trajectory, a 995-g fireworks rocket is moving horizontally at 18.6 m/s. It's a
dud, and instead of exploding gloriously, it bursts
into two pieces. One of them, with mass 372 g,
continues in the original direction at 31.3 m/s.

ill do it for 100, also this isnt 80 its 40

their crystal patterns have been disrupted by stress

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.