A group of kids come across a covered bridge that is a great spot for jumping off, but they want to make sure they stay safe so they want to check how high the
bridge is before anyone jumps off. They find a 5kg rock and guess that it will
experience a coefficient of friction of 0.21 as it falls. Once the rock is thrown over
the edge of the bridge, it takes 1.6s to reach the water.
How tall is the bridge?

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.

this would be about 43%