A solid uniform cone, of base radius r and height 6r, has a similar smaller cone of height 2r removed from the top to form a frustum. This frustum is placed, with its larger plane face, on a rough surface that is hinged to the floor at one edge. The surface is slowly rotated so that the incline angle increases. Given that the coefficient of friction is 0.85, find whether the frustum breaks equilibrium by toppling or sliding.