Justin is arranging a spring between two cubes on a glass tabletop. One cube is made of 2.5 kg of glass and the other is made of 7.8 kg of steel. The coefficient of friction between glass and glass is 0.94 and the coefficient of friction between steel and glass is 0.19. If the spring starts out 20 cm long and has a spring constant of 250 N/m, how close can you get the two cubes before they start sliding?