1. The problem statement, all variables and given/known data 20: Two boxes are sitting on top of one another. The top box has a mass of 5kg and is connected to the wall by an unstretched spring, k=2000N/m. The bottom box is 10kg. the static friction coefficient between the 2 boxes is 0.5, the kinetic friction coefficient between the 2 boxes is 0.25. the static friction coefficient between the bottom box and the floor is 0.4, and the kinetic friction coefficient is 0.2. a. Draw Free Body Diagrams b. What is the force F that causes the boxes to move relative to each other? c. How much does the spring stretch when force F is applied to the bottom box before the boxes move relative to each other? 2. Relevant equations Force of a spring = -kx Force of friction = mu (static or kinetic) times normal force Sum of forces = ma 3. The attempt at a solution I had no problem with part a For parts b and c I got about half way done and then found a problem. For part b, at first I assumed that for the two blocks to move relative to each other, the bottom block must already be moving so I should use the coefficient of kinetic friction. However, after I actually did the math out, I found that the force required to to overcome the first static force and make both blocks move together is greater than the sum of the two forces (kinetic friction of both, and static friction between the two) required to make them move relatively. So wouldn't that mean that the moment the lower block begins to move the two separate and so therefore the answer to part c is zero? Is this what occurs, or am I just misunderstanding the problem? I finished the problem ignoring that issue and got as answer for b and c 54 newtons and 1.2 centimeters. Are these right in that case? Thanks for the help?