Abstract

A diagnostic instrument consisting of 18 items was developed to evaluate students’ understandings about mechanics. The instrument was administered to 240 eleventh grade students attending four typical public high schools in Thailand. Analysis of the students’ responses identified the existence of patterns of reasoning that are common to conceptions in different areas of mechanics involving the direction of force acting on a moving object, objects moving under the sole influence of gravity, motion involving mechanical energy and momentum. Reliability proved sufficient (Cronbach’s alpha equals .7). The study revealed students held a difficulty in understanding why and when to use force, motion, energy and momentum concepts, and especially the qualitatively interpreting basic principle related to conservation laws. Their skills of problem analysis and judgment on the efficiency of a given solution to a problem were poor. We also found a mismatch between students’ confidence in answering the items and their correct responses through the correlation between the students’ confidence of being correct on each item and the actual correct responses being significantly less than 0.278. Thus this instrument is a convenient diagnostic tool that any teacher could use to identify students’ perceptions to the introduction of the topic and students’ alternative conceptions during classroom instruction.