What Do Students Perceive during a Lesson on Center-of-Mass?

This paper presents an assessment of how students perceive a typical classroom representation of center-of-mass. Participants were shown figures consisting of two filled black circles; they were told that these figures represented spheres or balls. They were then asked to indicate with a mark the point where the spheres would balance, i. e., they were asked to find the center-of-mass. There were two participant groups. The first group (labeled "Control") received only brief, written instructions on the exercise. The second group (labeled "COM") was shown a 15-minute video which discussed, qualitatively, the topic of center of mass and the same written instructions. They also received the same written instructions as the Control group. The results indicate how students perceive the problem. In all groups students do not estimate the center-of-mass as if they were considering spheres. We find that while the performance for both groups was poor, there is a difference between the two. This suggests some insight on teaching the topic. (Contains 3 figures.)