Two blocks rest on a horizontal frictionless surface as shown. The surfaces between the top and bottom blocks is roughened so that there is no slipping between the blocks. A 30-N force is applied to the bottom block as shown in the figure.

1. Determine the acceleration of the two-block "system"
2. What is the force of static friction betwen the top and bottom blocks?
3. What is the minimum μ_s necessary to keep the top block from slipping on the bottom block?