4. A car of mass 1200 kg travelling at 20 m/s brakes and comes to rest in 4 seconds. What is the average force exerted by the brakes on the car?*

The car has an average acceleration of

Then, by Newton's second law, the average force exerted by the brakes on the car is

which is to say, the average force has a magnitude of 6000 N and is directed opposite the car's motion.

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.