A 16.0-N block rests on a horizontal surface. The coefficients of static and kinetic friction between the surface and the block are 0.880 and 0.640, respectively. A horizontal string is attached to the block and a constant tension is maintained in the string. What is the force of friction acting on the block if the tension is 13.08 N? What is the force of friction acting on the block if the tension is 18.08 N?