Ah! I mixed it up again, I didn't think of dx as a distance, so I added an extra x to get some kind of work to sum up. It all makes sense now, thank you.
When I calculate what I got there, I get the answer 0.96680 Joule, which is NOT correct. That's what I do not understand! The correct answer is 2.58 Joule. It makes no sense.
So the total force on the small block is F_{2}=\frac{12}{9.82}*a, which is the same as (12 - T) N.
This means that the tension in the rope is T = 12 - \frac{12}{9.82}*a
This force T is the force that sets the large block in motion (?). The total force on the large block is then F_{1} = T -...
Well, the static friction is f_{s}=0.5*20 N, which is just 10 N. The small block pulls on the large one with a force of 12 N (am I right in assuming that?), so everything should be set in motion.
After that, the kinetic friction is all that works against the 12 N from the small block. Shouldn't...
The problem and solutions: http://i.imgur.com/hxlfbmZ.png
I was able to solve the first two questions, but I guessed part C.
For the first one, I thought of the two blocks as one unit (since there's no friction or air resistance). If they're both falling with the same acceleration, there...