An experiment in rotational motion with linear and quadratic drag

We describe a study of velocity-dependent drag in rotational motion that is suitable for an undergraduate laboratory experiment. Using standard teaching-laboratory equipment to obtain the data, we found that a drag force that is linear in the angular speed describes the data very well; however, the model residuals reveal that quadratic drag is also present. When a combined model is used, the residuals are reduced to the level of measurement uncertainties. An investigation of the effect of airflow on the air bearing used in the experiment confirms that there is a turbine effect on the disk and that the drag is dependent on the airflow suspending the disk. The experiment is simple to perform and analyze, yet reveals interesting physical and analytical complexity with deeper investigation. © 2004 American Association of Physics Teachers