An ideal spring is welded at one end to the smooth surface of a spin coater. The spring rests horizontally on the surface and has an equilibrium length of 14 cm. A small object of mass 55 g is fixed to the free end. The spin coater is rotated at a speed of 125 rpm. Once the object moves in a circular path at a constant speed, the spring length becomes 18 cm. Calculate the spring constant.