The purpose of the research is to study the frequency of harmonic oscillation with restoring force propotional to quadratic (F -kx2) and cubic (F - zeta x3) of the displacement. we also describe how the Atwood machine intrerfaced to a personal computer trought a rotary encoder is suited for investigating harmonic oscillation exploiting the buoyancy force acting on a body immersed in water. The result of this research theoreticalliy and experimental can be concluded that twin triangles would oscillate with restoring force propotional to the square of its displacement and its frequency linear to the root of its amplitude The learning activity of the harmonic oscillation was performed using demonstration and expriment method. The purpose of this learning was to determine periodesation of harmonic pscillation. Futher more students could using process skill and guide approach.