Astronomers are always making comparisons between measurements in astronomy and something that might be more familiar. for example, the hipparcos web pages tell us that the measurement accuracy of 0.001 arcsec is equivalent to the angle made by a golf ball viewed from across the atlantic ocean, or to the angle made by the height of a person on the moon as viewed from earth, or to the length of growth of a human hair in 10 sec as seen from 10 meters away. use the ideas in example 19.2 to verify one of the first two comparisons.

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.