This paper is divided in primary chapters and problem solving chapter. Chapter „introduction“ gives some basic ideas which were discussed in paper. Chapters 2-5 discussed theoretical and mathematical models which are used to understand problem and given task for solving. In primary chapetrs we saw how science and engineering progressed from Newton's second law which was used to describe vibrations, also we mentioned Jeffcot's rotor model and everything up to finite element method. Problems with vibrations in mechanical engineering were mentioned, their examples, also it was given insight into one degree of freedom vibrations. Jeffcot's rotor model was explained, model was very important for understanding problem solving chapter. Problems with rotating and nonrotating disk was explained, also important for this paper was to mention and give some basic assumptions about finite element method. It's good to mention that simulations would not be able to be done without finite element method. Paper was described with equations, and pictures and graphs where possible. In chapter 6. problem was solved, problem was to determine first five natural frequencies of circular disk, which was clamped in centre, shaft was supported with two bearings. Analysis was solved for three characteristic examples, Campbell's graphs were made.