When I say unsolvable, I mean that you can't solve for the speed of a brick sliding down an inlined plane by considering the electrostatic interactions between

When I say unsolvable, I mean that you can't solve for the speed of a brick sliding down an inlined plane by considering the electrostatic interactions between the ~10^17 pairs of interacting atoms per square centimetre of contact surface. It's just unsolvable in practice. But the effective formula that the magnitude of the friction force equals a material constant times the magnitude of the normal force from the plane onto the brick gives you a very simple model (solvable with pen and paper).