This question has been posted and I do apologize for posting again, but I am having trouble determining when to use Fnet=ma or Fnet= 0 for questions like these. I have solved it but need help interpreting the question. "At a construction site, a small crane is raising two boxes of nails on a plank to the roof. One box has already been opened and is half-full, while the other box is new. The boxes, including the nails, weigh 10kg and 20kg respectively, and are the same size" "b) If the coefficient of static friction is 0.4, draw and FBD for each box of nails and use it to calculate the angle at which each box begins to slide". So here, I am getting confused. To me, I am seeing this motion involving acceleration as it is beginning to move. But in order to answer, the problem does not involve Fnet=ma. Why is this? "c) If the coefficient of kinetic friction is 0.3, how fast will the boxes accelerate along the plank once they start to slide?" So now, we are involving acceleration, but in b) we are not, and it appears that the language is the same to me (ie. beginning to move vs. starting to slide). It just seems like they are saying the same thing. Could someone please help me understand this? Thank you!