Physics Seminar
Wednesday, August 25, 2004, 4:30 pm
Willet Science Center 101

Randall D. Peters
Department of Physics
Mercer University

Debunking the Linear Damping Model for Mechanical Harmonic Oscillators

Earlier experiments as a function of frequency suggested that internal friction damping of mechanical oscillators may be significantly nonlinear.  As such, experiment would be in violation of the widespread belief that damping friction is proportional to velocity (commonly called viscous damping).  In a recent set of experiments, a folded pendulum was driven to steady state at drive frequencies below resonance.  Motion at the natural frequency was observed not to decay toward zero in transient manner, as required by the common theory.   It was instead observed to persist at levels significantly above noise, when the frequency of the sub-resonance drive was 1/n, where n = odd integer.  Although this behavior cannot be explained by linear damping models, the figure below shows that it can be explained by the nonlinear modified Coulomb model developed by the author.  Also observed was a surprisingly large distortion in the steady state response, for drive frequency = 1/2.  Shown to be independent of drive intensity, this distortion can be explained by anisotropy ― different friction forces for opposite directions of the motion.  Moreover, for agreement between the nonlinear theory and experiment, the damping must be negative during one set of half-cycles of the motion.

Please join us for light refreshments at 4:15pm outside WSC 109.