Above is a roller coaster. There are 5 points of interest. (1) We are interested in the maximum height that the loop, point C, can be and the coaster still make it around the loop in terms of the height of point A. Assume no friction. (2) Now friction plays a role. If 1/3 of the energy is lost to friction between points A and C, now how high will point C be to make it around the loop in terms of point A? (3) If friction is involved, how do the speeds at points B and C compare? (4) If 1/3 of energy is lost between points C and E, and the car is to stop moving once it reaches point E, how high will point E be in terms of the height of point A?

Explanation:

1) if there is no frictions , them P.E of A  converted to K.E of B and this is again converted to C.

So, height of C  = height of A

2) ENERGY AT A = E = mghA

Energy at C = E/3 = mghc

mgh A = 3mghC,

therefore, HC = HA/3

3) POINT B has maximum velocity

point C has minimum velocity.

4) Energy at C = E1= mghC  

energy at E = E 1/3 = mghE

therefore ,

hE = hC/3

think its the third option

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.