No abstract

Participants estimated the perceptual center of mass between two horizontally oriented black dots varying in size and distance. Experiment 1 showed that estimates,measured as distance from the larger dot's center, decreased with an increase in size ratio between the dots and a decrease in the distance between them, as predicted by the physical center-of-mass equation. The results were replicated and extended in further experiments with different ratios and distances. In all experiments, the true center was consistently overestimated, either because the stimuli were perceived in a low dimensionality or because of a hesitancy to place estimates near the larger dot's edge. In Experiment 4, the center was deliberately placed near this edge for one-and two-dimensional solutions. Participants still overestimated, indicating an "edge effect" as responsible.

When most materials are heated they expand.1 This concept is usually demonstrated using some type of mechanical measurement of the linear expansion of a metal rod.2 We have developed an alternative laboratory method for measuring thermal expansion by using a Michelson interferometer. Using the method presented, interference, interferometry, and the principle of thermal expansion can be taught concurrently. The material is accessible to undergraduates and advanced high school physics students.

Participants judged the perceived center of three-dot displays at different orientations. In Experiment 1, the dots formed an equilateral triangle. The direction of the response distribution aligned with the largest dot, along axes of reflectional symmetry and with the gravitational down. In Experiment 2, we created isosceles triangles where the distance between one of the dot pairs was varied. Errors were higher for configurations where the symmetry axis was horizontal and a virtual elongation axis was vertical. The results of Experiment 3 replicate this finding and show that response direction is an accommodation to gravity and other shape factors.

Center-of-mass perception in dot patterns arranged into triangular and quadrilateral configurations was investigated. In Exp. 1, the length, orientation, and direction of dots forming right triangles were varied. Errors in center-of-mass perception by 19 participants increased with the length of the triangles. In Exp. 2, 27 participants estimated the centers of squares and rectangles made up of four dots. Accuracy was best for squares that contained greater reflective and rotational symmetry. The orientation of responses was generally downward in both experiments, indicating a gravitational influence. The results are consistent with a process model by which observers use internal pattern structure like medians and axes when estimating the center of an object.