Choose Wisely: Static or Kinetic Friction—The Power of Dimensionless Plots

摘要

Consider a problem of sliding blocks, one stacked atop the other, resting on a friction-less table. If the bottom block is pulled hori-zontally, nature makes a choice: if the applied force is small, static friction between the blocks acceleratesthe blocks together, but with a large force the blocksslide apart. In that case, kinetic friction still forcesthe upper block forward but with less accelerationthan the lower block. The choice, then, lies in therelative terms—what is meant by small and large?After a confusing experience during a recent exam,we've found a demonstration and graphical presen-tation that can help clarify the distinction betweenstatic and kinetic friction.