Block A, with mass mA, is initially at rest on a horizontal floor. Block B, with mass mB, is initially at rest on the horizontal top surface of A. The coefficient of static friction between the two blocks is μs. Block A is pulled with a horizontal force. It begins to slide out from under B if the force is greater than: A. mAg B. mBg C. μsmAg D. μsmBg E. μs(mA+mB)g I came up with a couple of equations (that may not be right). f = μmBg f = F - mAaA So I tried substitution and solving for F, but I can't get the right answer (E). So what's the right way to do this problem?