This dialog considers speed, a fundamental concept encountered by students at the beginning of a physics course. The concept depends on earlier ideas of distance and time, but it demands ability with ratio reasoning.
Students come into beginning physics courses with a variety of naive misconceptions about the concepts involved. For speed these concepts have been examined in studies at the University of Washington, under the direction of Lillian McDermott. Speed uses the results of these studies in deciding new examples to consider.
The dialog begins with an experiential situation, to give students practice with speeds. They are asked to carry out a variety of tasks involving moving balls, with aid if necessary, until they conduct them successfully.
We then consider situations in which the time is held fixed, but the distances vary. Then we consider examples where the distance is fixed but the time varies. Finally we allow both to vary, with students measuring each.