In the analysis or every engineering problem, we must represent the system under scrutiny by a physical model. We may often represent a continuous or distributed parameter system (one in which the mass and spring elements are continuously spread over space) by a discrete or lumped-parameter model, one in which the mass and spring elements are separate and concentrated). The resulting simplified model is especially accurate when some portions of a continuous system are relatively massive in comparison with other portion For example, the physical model of a ship propeller shaft is often assumed to be a massless but twistable rod with a disk rigidly attached to each end-one disk representing the turbine and the other representing the propeller. As a second example, we observe that the mass of springs may often beneglected in comparison with that orattached bodies.