Find the mass of an object on earth that weighs 39 N?

3.9kg

Explanation:

From newton's second law of motion

F =ma

a = 10m/s^2

F=39N

a= F/m

a= 39/10

a= 3.9m/s^2

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.