A 27.0-kg block is resting on a flat horizontal table. On top of this block is resting a 13.0-kg block, to which a horizontal spring is attached, as the drawing illustrates. The spring constant of the spring is 310 N/m. The coefficient of kinetic friction between the lower block and the table is 0.490, and the coefficient of static friction between the two blocks is 0.720. A horizontal force 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.
Determine the amount by which the spring is compressed at the point where the upper block begins to slip on the lower block.
i just don't know if the coefficients of friction will be pointing towards the wall or away