A common lab exercise in the introductory college physics course employs a low-friction cart and associated track to study the validity of Newton's second law. Yet for college students, especially those who have already encountered a good high school physics course, the exercise must seem a little pointless. These students have already learned to accept Newton's laws without question, and any experimental data that contradict the second law would immediately alert students to an error in procedure or analysis, or, worse, reinforce the widely held opinion that simple laws are inadequate to explain the behavior of "real" systems. A better approach is to ask students to apply their understanding of Newton's laws to determine one or more unknowns inherent in the laboratory apparatus. We illustrate this approach in the experiment described below: a small amount of complexity is added to a standard experimental exercise, forcing a careful analysis of the collected data and yielding very accurate results plus a thorough understanding of the physical system under study. If development of experimental skills is one of the primary goals of the introductory laboratory, then the strategy illustrated below might be widely adaptable and appropriate in laboratories throughout the introductory mechanics curriculum.