Among the student ideas about forces discussed in the literature, perhaps the most commonly reported is the notion of an impetus force, or the "belief that there is a force inside a moving object that keeps it going and causes it to have some speed." For example, Clement asked university students taking introductory mechanics to draw a free-body diagram for a coin that has been tossed upward. He found that students often drew an arrow in the direction of the coin's motion, at a point midway between the initial toss and the turnaround point, sometimes providing reasoning that suggested that the arrow corresponds to a "force from your hand" or the "force of the throw." Clement interpreted these responses as indicating that "student[s] may believe that continuing motion implies the presence of a continuing force in the same direction, as a necessary cause of the motion." As another example, in a study conducted with undergraduate students at Johns Hopkins University, McCloskey, Caramazza, and Green asked students to draw the path a ball will follow after it exits the curved tubes pictured in Fig. 1. The authors found that students often drew curved trajectories for the ball exiting each tube, and they coined the term "curvilinear impetus principle" to describe students' reasoning. They write that students reasoned as though "an object constrained to move in a curved path acquires a curvilinear impetus that causes it to continue in a curved trajectory for some time after the constraints on its motion are removed."