A 30.0-kg block is resting on a flat horizontal table. On top of this block is resting a 15.0-kg block, to which a horizontal spring is attached, as the drawing illustrates. The spring constant of the spring is 340 N/m. The coefficient of kinetic friction between the lower block and the table is 0.640, and the coefficient of static friction between the two blocks is 0.870. A horizontal force vector F is applied to the lower block as shown. This force is increasing in such a way as to keep the blocks moving at a constant speed. At the point where the upper block begins to slip on the lower block determine the following.