Abstract: Two unexpected phenomena got from the experiment ＇ torsion pendulum of free and forced rotational oscillations using the Poht resonance equipment are analyzed. Mechanicl damping and nonlinear effects of coil spring are found to be mainly responsible for the unexpected phenomenon. Furthermore, unstable driving fre- quency under some conditions can be the possible explanation for the second unexpected phenomenon. The linear dynamical equations are modified to become a nonlinear equation whose numerical solution explains the unexpected phenomena successfully.

Key words: Pohl resonance torsion pendulum, nonlinear physics, numerical ealeulation