Soup-can Pendulum

Abstract

In these studies, a vegetable can containing fluid was swung as a pendulum by supporting its end-lips with a pair of knife edges. The motion was measured with a capacitive sensor and the logarithmic decrement in free decay was estimated from computer-collected records. Measurements performed with nine different homogeneous liquids, distributed through six decades in the viscosity η, showed that the damping of the system is dominated by η rather than external forces from air or the knife edges. The log decrement was found to be maximum (0.28) in the vicinity of η = 0.7 Pa s and fell off more than 15 fold (below 2 x 10⁻²) for both small viscosity (η < 1 x 10⁻³ Pa s) and also for large viscosity (η > 1 x 10³ Pa s). A simple model has been formulated, which yields reasonable agreement between theory and experiment by approximating the relative rotation of can and liquid.