1. The problem statement, all variables and given/known data Both the question and solution are on this link (Question 7): http://sin.uwaterloo.ca/solutions_2012.pdf I understand the solution they provided, but I think the question is misleading and should not have been worded that way. I just want to clarify if i'm right or not. 2. Relevant equations 3. The attempt at a solution I initially thought that no matter what the spring was doing in the middle, the force applied must be the sum of the frictions of both boxes, or 2umg. If the boxes were connected I know the force would definitely be 2umg, and it seemed illogical that adding a little spring would give me free energy. In case the handwriting is hard to read: Wtotal = Wspring + Wbox1 Ftotal * x = 0.5kx^2 + Fbox1 * x Ftotal = 0.5kx + Fbox1 Then they said that since that the force applied by the spring is equal to the force of friction on the box2: kx = umg Ftotal = 0.5umg + umg = 1.5 umg I don't think it's fair to say that the force I need to apply is 1.5umg because I can rewrite: Wspring = Faverage * x therefore Ftotal = Faverage + Fbox1 So shouldn't Ftotal just be the average force that I need to apply, and not the full force once the spring is fully stretched? If I can only apply 1.5umg of force and no more, shouldn't I not be able to move the box since I can't apply the full 2umg for the spring to stretch fully? Thank you for reading.