Direction: The order of a system is defined as being the highest power of derivative in the differential equation, or being the highest power of s in the denominator of the transfer function. A first-order system only has s to the power one in the denominator. while a second-order system has the highest power of s in the denominator being two. Many system are approximately first-order. The important feature is that the storage of mass, momentum and energy can be captured by one parameter. Examples of first-order systems are velocity of a car on the road, control of the velocity of a rotating system, electric systems where energy storage is essentially in one capacitor or one inductor, in compressible fluid flow in a pipe, level control of a tank, pressure control in a gas tank, temperature in a body with essentially uniform temperature distribution (e.g., steam filled vessel).