I'm working on this problem for homework. Basically it's to find the work done by friction. Now for the work it would be the force of friction times the displacement. The problem is the board being pushed is going over 2 different surfaces, with 2 different values for friction, at the same time. Assuming velocity of the board is constant, what would be the force of friction? I thought it was the normal force times the sum of the coefficients (F_friction=N(mu_1+mu_2)) but apparently it's not.