Serway & Jewett — Rotation of a Rigid Object about a Fixed Axis: Problem 19

Your grandmother enjoys creating pottery as a hobby. She uses a potter’s wheel, which is a stone disk of radius \(R = 0.500\text{ m}\) and mass \(M = 100\text{ kg}\). In operation, the wheel rotates at \(50.0\text{ rev/min}\). While the wheel is spinning, your grandmother works clay at the center of the wheel with her hands into a pot-shaped object with circular symmetry. When the correct shape is reached, she wants to stop the wheel in as short a time interval as possible, so that the shape of the pot is not further distorted by the rotation. She pushes continuously with a wet rag as hard as she can radially inward on the edge of the wheel and the wheel stops in \(6.00\text{ s}\). (a) You would like to build a brake to stop the wheel in a shorter time interval, but you must determine the coefficient of friction between the rag and the wheel in order to design a better system. You determine that the maximum pressing force your grandmother can sustain for \(6.00\text{ s}\) is \(70.0\text{ N}\). (b) What If? If your grandmother instead chooses to press down on the upper surface of the wheel a distance \(r = 0.300\text{ m}\) from the axis of rotation, what is the force needed to stop the wheel in \(6.00\text{ s}\)? Assume that the coefficient of kinetic friction between the wet rag and the wheel remains the same as before.