What is critical damping for a spring, dashpot and mass vibrating system?

Question

.(i)     What is critical damping for a spring,    dashpot and mass vibrating system?

(ii)   Such a system has mass m = 10 kg, spring stiffness k = 4000 N/m and damping coefficient c = 40 N/m/s. Find the critical damping coefficient, and the damping factor .