We solve the classical problem-model of the oscillation of the gravitational pendulum on the elastic thread. This task was formulated, but it is not completely solved in the classical K. Magnus
monograph “Wavering”. With additional assumptions about the analyzed model in question (mass
conservation law and considerations about certain physic) system of the differential equations of the
two second-order nonlinear oscillation transformed into a third order nonlinear differential equation
which is solved numerical approximate using the Maple program. The solution is compared with
asymptotic in a long time range.