Last post ended with a question: when a physicist plays with clocks and rods and kinematic concepts like time, displacement, velocity, acceleration… what is his or her Cinderella, that is to say, the practical purpose that he or she has in mind?

Seeking an illustration for the answer, I reviewed the problems posed in a standard physics book. Unfortunately, the questions stopped at the abstract level. They asked, for example, about the velocity of a projectile after the lapse of a...