A control system in which the dynamics are determined by second-order differential equations. They mainly arise in process plants as the result of the combination of two first-order systems in series, the consequence of the addition of complex control systems, or the system may itself be intrinsically second order. This is generally uncommon where only the continuity equations are needed for process models such as mass, component, and energy balances which are inherently first order. If continuity equations for momentum are needed, the resulting balances will be inherently second order. Such balances are used for the modelling of mechanical systems such as control valves or the behaviour of the fluid dynamics of a process.