Academic journal article Science Educator

Weight, Mass, and Gravity: Threshold Concepts in Learning Science

Academic journal article Science Educator

Weight, Mass, and Gravity: Threshold Concepts in Learning Science

Article excerpt

Introduction

Large parts of our nations' science education systems are broken, and we have known about some of the problems through educational research for decades (U.S. National Commission on Excellence in Education, 1983; Fleishman, 2010; Martin et. al, 2012). It's time to bring these findings to a broader audience so that practitioners can use the information to fix some of the most glaring problems. A recent step in the right direction has been development of Next Generation Science Standards (NGSS Lead States, 2013) in the United States, but it is just that-a step. The new standards provide clear targets for assessment, but do not provide a pathway for reaching them.

In this paper, we argue that to meet the new standards curriculum developers and teachers must focus on helping students develop deep, flexible, and useful threshold concepts that provide the intellectual underpinnings of the standards. To illustrate this instructional strategy we have chosen the concepts of weight, mass, and gravity, since they are essential for grasping a number of performance expectations in all disciplinary areas. While we understand and support the new vision for threedimensional instruction (i.e., combining practices, crosscutting concepts, and core ideas) implicit in the NGSS, we claim that the need to help children grasp fundamental concepts continues to be important.

The difficulties posed by students' inability to understand and differentiate the concepts of weight and mass are well known in physics. Klopfer, Champagne, and Chaiklin (1992) noted that children enter school with initial ideas about certain essential "ubiquitous quantities" (e.g., weight, mass, volume and density), and that a goal of science instruction is to develop the spontaneous concepts into a scientific understanding of these quantities-a goal that the authors acknowledged was only rarely achieved, even by the time students reach college.

Ina compelling video, entitled "Lessons from Thin Air" (Schneps & Sadler, 1997), a bright seventh grader is interviewed before and after a class on photosynthesis. Prior to the class he is asked what a tree is mostly made from. His response is that it is made from soil and water. After a six-day unit on photosynthesis, during which he experienced a lab activity coordinated with lectures on how plants extract carbon from carbon dioxide in the air, and learns about the chemistry of photosynthesis, he was again asked the same question. Despite instruction from a knowledgeable teacher, he again gives the same answer-that the material in a tree comes from soil and water. Although he correctly stated the chemical equation for photosynthesis, when asked if the wood, bark, and leaves come mostly from carbon dioxide in the air, he replies that is impossible because air doesn't weigh anything, stating "If it did we wouldn't be able to breathe." Although it may not be surprising that seventh grade students have difficulty understanding that gases have weight, the same video shows that graduates from Harvard and MIT give the same answer as the high school student-that the considerable mass of a tree could not possibly have come from carbon in the air. Like the seventh grader, the college graduates had no difficulty memorizing the chemical formula for photosynthesis, but when they really thought about it, even that classic example of conservation of mass seemed like they were getting something (wood, bark, and leaves) from nothing (the air.)

The third threshold concept that we will describe in this paper is the meaning of gravity, which also seems to be poorly learned at the middle school level (Kavanagh and Sneider, 2007a, 2007b), with misconceptions continuing into adulthood. Recognizing that understanding of basic physics concepts are important for the study of geology, college professors Ashghar and Libarkin (2010) surveyed 197 students enrolled in geology courses at a mid-western university concerning their understanding of gravity. …

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