Three identical spheres lie in contact with one another on a horizontal plane. A fourth sphere rests on them, touching all three. Show that, in equilibrium, the coefficient of friction between the spheres is at least (rad(3)-rad(2)) and that the coefficient between each sphere and the plane is at least (rad(3)-rad(2))/4 The only thing I can think to use here is summing forces. But woah, I am so stuck. Do I have the right idea even? Any help would be appreciated it! Thanks!