A mechanical system which can be modelled by a spring, dash pot and mass is an example of a second order system. When we apply a load to the system then oscillations occur which have amplitudes which die away with time. This was illustrated in the opening section and Figure 2.2. Likewise with the second order system of an electrical circuit having resistance, inductance and capacitance; when there is a step voltage input, i.e. a switch is closed and applies a constant voltage to the circuit, then the voltage across the capacitor will be described by a second order differential equation and so can oscillate with amplitudes which die away with time.