A 5 kg block sits on top of a 10 kg block which rests on a horizontal frictionless surface. The surface between the top and bottom blocks is roughened so that there is no slipping between the two blocks. A 30-N force is applied to the bottom block. What is the force of static friction between the top and bottom block? What is the minimum coefficient of static friction necessary to keep the top block from slipping on the bottom block?