Detection of Violations of the Law of Pendulum Motion: Observers' Sensitivity to the Relation Between Period and Length

Abstract

Newtonian mechanics show that the period of a freely swinging pendulum is proportional to the square root of its length. Subjects' use of this relationship in perceiving the naturalness of motion was tested by presenting displays with periods physically correct, too short, or too long for the lengths of the pendulums. Sensitivity to violations of the length-period relationship was assessed by col- lecting ratings of the perceived naturalness of the motion. Deviations as small as .I to .2 sec were detected. The numerical ratings, however, varied widely among subjects and revealed a tendency to perceive physically correct motion as some- what more rapid than natural, The salience of violations of the length-period relationship was tested in a free response task asking subjects to comment on the appearance of the displays. Large deviations drew moderate numbers of observa- tions of the unnaturalness of the motion and a few statements about the percep- tion of forces acting on the pendulum. These comments show that some of the information granted by pendulum dynamics is detected by the perceptual system and allows reasonably accurate discrimination between pendulums swinging freely under the influence of gravity alone and those subject to additional forces. The results of these studies are in general agreement with those of Bozzi (1958) and Pittenger (1985).