Describe friction force (Ffr) in your own words

 Friction is like when u take two stick's and rub it together to make fire when you use friction it can produce heat. Applied force is like and applied to an object or person if a person is pushing a desk across the room then there is applied force.

the force required to move the cart is divided by three or 33.33 n.

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.