A 50 kg box is at rest on a horizontal surface. When an 80 N force is applied to the box, it slides on the surface with a constant velocity. How will the force of friction change if an identical box is placed on top of the 50 kg box

As the box is moving with a constant velocity, the two forces acting on the box are canceling each other.Then   friction force = 80 Newtons              but in the opposite direction. Friction force =  Mu  * Normal force exerted by ground  =  Mu * weight of boxSo we find Mu.Mu = coefficient of friction between box and horizontal surface            = Force of friction / weight  =  80 / 50 * 9.81 = 0.163 When an identical box is placed on top, the force of friction is       = Mu * total weight = 0.163 * (50+50) * 9.81 = 159.9 Newtons