Soup-can Pendulum

Abstract

In these studies, a vegetable can containing fluidwas swung as a pendulum by supporting its end-lipswith a pair of knife edges. The motion was measuredwith a capacitive sensor and the logarithmic decrementin free decay was estimated from computer-collectedrecords. Measurements performed with nine differenthomogeneous liquids, distributed through six decadesin the viscosity η, showed that the damping ofthe system is dominated by η rather than externalforces from air or the knife edges. The log decrementwas found to be maximum (0.28) in the vicinity ofη = 0.7 Pa s and fell off more than 15 fold(below 2 × 10^-2) for both small viscosity(η < 1 × 10^-3 Pa s) and also for largeviscosity (η > 1 × 10³ Pa s). A simple modelhas been formulated, which yields reasonable agreementbetween theory and experiment by approximating therelative rotation of can and liquid.