Solve the each part of given question:
Question: A 47 N block rests on a horizontal surface. The coefficients of static and kinetic friction between the surface and the block are 0.9 and 0.5, respectively. A horizontal string is attached to the block and a constant tension is maintained in the string.
Part A: What is the force of friction acting on the block if the tension is 12 N?
Part B: What is the force of friction acting on the block if the tension is 55 N?
Please give me an explanation for above questions.