One of the important problems of mathematical modelling of dynamic systems is the coincidence of modelling results with experimental measurements. Such a coincidence is being attained by construction of "correct" mathematical model (MM) of the dynamical system and the choice of a “good” model of external load (MEL). MM of object the motion of which coincides with experimental measurements with acceptable accuracy under action of MEL (or external impact) is understood by us as “correct” model. Thus the degree of "correctness" of MM depends directly on the chosen model of EL and required accuracy of the coincidence with experiment. It is formally possible to write as an inequality for models with the concentrated parameters the following: