A 23.0-N block rests on a horizontal surface. The coefficients of static and kinetic friction between the surface and the block are ?s=0.720 and ?k=0.630. A horizontal string is attached to the block and a constant tension T is maintained in the string. What is the force of friction acting on the block if T = 15.56 N? What is the force of friction acting on the block if T = 20.56 N?