In many situations, energy is lost to the work done by friction. But the law of conservation of energy says energy cannot be created or destroyed. So where does the "lost" energy go?

Heat energy

Explanation:

The seemingly lost energy due to the work done by friction is lost to heat energy.

When we account for heat energy of the system, it is clearly seen that it is not lost as we perceive it.

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.

i think the answer is a and b

i wish you luck