Often in the analysis of physical systems the response of the system to a particular test input is calculated and/or measured. Commonly used inputs include a step, and impulse, or a sinusoid. On occasion a more complicated input may be used, but in testing situations the three named functions are most commonly used. In most cases the test conditions are such that a zero state response is measured. The response of systems to the various inputs are each described on their own pages. In particular, the response of first and second order systems are discussed in some depths.