The motion of a particle in different potentials is investigated theoretically and experimentally. The dependence of the period of oscillation on the amplitude is studied for pendula associated with some of these potentials. A technique is proposed to modify the trajectory of a pendulum bob so that it moves along a predetermined curve, and a simple and low cost experiment to study the relation between the period and amplitude for different potentials is discussed. We report on the motion of several pendula whose periods decrease with increasing amplitude. In particular, we study the effects of a perturbation of the form z4 on the frequency of oscillation of a simple harmonic oscillator. Our results agree with the expectation that any perturbation of a simple harmonic oscillator destroys its isochronism.

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