The general equation of motion for a single degree of freedom (SDOF) mechanical system is $m\ddot{x} + c\dot{x} + kx = F(t)$. The nature of the vibration (free/forced, damped/undamped) is determined by the terms present: - If $F(t) = 0$, it's free vibration. If $F(t) \neq 0$, it's forced vibration. - If $c \neq 0$, it's vibration with damping. If $c = 0$, it's vibration without damping.