Chapter 5, Problem 70GP

* Determining the forces between powders and surfaces (adhesive forces) plays an important role in several industrial applications One method to determine adhesive forces employs the following procedure. Powder is deposited on a flat surface that was previously covered with a layer of the material under study, for example, glue. The sample is then spun very quickly for some time. During the spinning, some particles fly off the surface and some remain on it. Examining the sample after the rotation allows scientists to determine the maximum force that the surface exerted on the powder particles. (a) Explain the physics principles that are employed in this method. Figure P5.70a shows salt crystals (cubes of average mass 0.06 mg) stuck to a piece of cardboard by a layer of glue. Figure P5.70b shows the same piece of cardboard after it was spun at 80 rotations per second about the axis of rotation shown by the black dot in center of the photos. The yellow curve surrounds the area where the salt mostly remained on the cardboard. (b) Estimate the maximum force exerted by the glue on a salt crystal in this experiment. Indicate any assumptions that you made. Note: You will need a ruler to solve this problem!