A block of mass m is at rest at the origin at t = 0. It is pushed with constant force F_0 from x = 0 to x = L across a horizontal surface whose coefficient of kinetic friction is mu_k = mu_0 (1-x/L). That is, the coefficient of friction decreases from mu_0 at x = 0 to zero at x = L. a. Use what you’ve learned in calculus to prove that a_x = v_x dv_x/dx b. Find an expression for the block’s speed as it reaches position L.