Below is a diagram of a weight on a spring. when the weight is pulled down and then released the spring compresses and expands. each position indicates a different point in time.
a. in all four positions mechanical energy is 0 j
b. the elastic potential energy of the spring at position 2 is converted to kinetic energy
c. the chemical potential energy of the spring is greatest at position 4
d. in all four positions, the gravitational potential energy of the spring is the same

seeing the graph would

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.

the answer is 12 as i may remember

: )