A small motor, of mass \(50 \mathrm{~kg}\), is found to have a natural frequency of \(100 \mathrm{rad} / \mathrm{s}\). It is proposed that an undamped vibration absorber of mass \(4 \mathrm{~kg}\) be used to suppress the vibrations when the motor operates at \(80 \mathrm{rad} / \mathrm{s}\). Determine the necessary stiffness of the absorber.