Varda Bar, Yaffa Brosh, and Cary Sneider Weight, Mass, and Gravity: Threshold Concepts in Learning Science Abstract information to fi x some of the most glar- a bright seventh grader is interviewed Threshold concepts are essential ideas ing problems. A recent step in the right before and after a class on photosynthe- about the natural world that present ei- direction has been development of Next sis. Prior to the class he is asked what a ther a barrier or a gateway to a deep un- Generation Science Standards (NGSS tree is mostly made from. His response derstanding of science. Weight, mass, Lead States, 2013) in the United States, is that it is made from soil and water. and gravity are threshold concepts that but it is just that—a step. The new stan- After a six-day unit on photosynthe- underpin students’ abilities to understand dards provide clear targets for assess- sis, during which he experienced a lab important ideas in all fi elds of science, ment, but do not provide a pathway for activity coordinated with lectures on embodied in the performance expecta- reaching them. how plants extract carbon from carbon tions in the Next Generation Science In this paper, we argue that to meet dioxide in the air, and learns about the Standards (NGSS). This study begins the new standards curriculum develop- chemistry of photosynthesis, he was with a review of research on students’ ers and teachers must focus on help- again asked the same question. Despite diffi culties in understanding these con- ing students develop deep, fl exible, and instruction from a knowledgeable teacher, cepts individually and in relation to each useful threshold concepts that provide he again gives the same answer—that the other, based on individual interviews and the intellectual underpinnings of the material in a tree comes from soil and surveys of several hundred children that standards. To illustrate this instructional water. Although he correctly stated the illustrate how students’ understanding strategy we have chosen the concepts chemical equation for photosynthesis, of weight, mass, and gravity develops of weight, mass, and gravity, since they when asked if the wood, bark, and leaves over the lifespan, from age fi ve through are essential for grasping a number of come mostly from carbon dioxide in the adult. New data from an additional 451 performance expectations in all dis- air, he replies that is impossible because subjects in the critical age range of 10 ciplinary areas. While we understand air doesn’t weigh anything, stating “If to 14 years old support and extend the and support the new vision for three- it did we wouldn’t be able to breathe.” prior fi ndings. The purpose of the cur- dimensional instruction (i.e., combin- Although it may not be surprising that rent study is to provide teachers and ing practices, crosscutting concepts, seventh grade students have diffi culty curriculum developers with actionable and core ideas) implicit in the NGSS, understanding that gases have weight, and up-to-date information that educa- we claim that the need to help children the same video shows that graduates tors can use to help children at the up- grasp fundamental concepts continues from Harvard and MIT give the same per elementary, middle, and high school to be important. answer as the high school student—that levels achieve Next Generation Science The diffi culties posed by students’ in- the considerable mass of a tree could not Standards. ability to understand and differentiate the possibly have come from carbon in the concepts of weight and mass are well air. Like the seventh grader, the college Introduction known in physics. Klopfer, Champagne, graduates had no diffi culty memorizing Large parts of our nations’ science and Chaiklin (1992) noted that children the chemical formula for photosynthe- education systems are broken, and we enter school with initial ideas about sis, but when they really thought about have known about some of the problems certain essential “ubiquitous quanti- it, even that classic example of conserva- through educational research for de- ties” (e.g., weight, mass, volume and tion of mass seemed like they were get- cades (U.S. National Commission on density), and that a goal of science in- ting something (wood, bark, and leaves) Excellence in Education, 1983; Fleishman, struction is to develop the spontaneous from nothing (the air.) 2010; Martin et. al, 2012). It’s time concepts into a scientifi c understand- The third threshold concept that we to bring these fi ndings to a broader au- ing of these quantities—a goal that will describe in this paper is the mean- dience so that practitioners can use the the authors acknowledged was only ing of gravity, which also seems to be rarely achieved, even by the time stu- poorly learned at the middle school level Keywords: middle school, standards, physical dents reach college. (Kavanagh and Sneider, 2007a, 2007b), science, weight, mass, gravity In a compelling video, entitled “Lessons with misconceptions continuing into adult- from Thin Air” (Schneps & Sadler, 1997), hood. Recognizing that understanding 22 SCIENCE EDUCATOR of basic physics concepts are important climate and the interaction of Earth falling, or being thrown upwards. This is for the study of geology, college profes- systems. also an area that has been widely stud- sors Ashghar and Libarkin (2010) sur- 5-PS1-2. Measure and graph quan- ied, but for which some questions still veyed 197 students enrolled in geology tities to provide evidence that re- remain (Agan & Sneider, 2004). courses at a mid-western university con- gardless of the type of change that The present investigation has been cerning their understanding of gravity. occurs when heating, cooling, or undertaken to provide a fi ner-grained anal- They found that only 21% had the cor- mixing substances, the total weight ysis of students’ schemas during the crit- rect scientifi c idea that gravity is a force of matter is conserved. ical period ages 10 to 14, when students of attraction, and that very few students are expected to develop an initial scien- incorporated the concepts of attraction, The concept underpinning this perfor- tifi c understanding of gravity, weight, mass, and force into their explanations mance expectation is conservation of and mass and their interconnections. of gravity. Various students expressed weight under phase transformations, Although there is an extensive litera- the ideas that Earth’s “spin,” “magnetism,” which can be very challenging for fi fth ture on students’ understanding of these and “atmosphere” caused the force of grade students (Galili & Bar, 1997), as threshold concepts, most of the studies attraction between Earth and other ob- we will discuss in our literature review. were done decades ago, so the fi ndings jects. The researchers identifi ed gravity MS-PS2-2. Plan an investigation to may not be true of today’s students. By as one of a small group of important provide evidence that the change in conducting a systematic study of chil- threshold concepts that can act as bar- an object’s motion depends on the dren’s ideas across a broad age range riers to learning, or provide a gateway sum of the forces on the object and we hope to provide teachers and curricu- to deep understanding of many related the mass of the object. lum developers with the actionable and concepts. Concepts underlying this performance up-to-date information that they need to In this paper, we have chosen to focus expectation include mass, as embodied help students at the upper elementary, on weight, mass and gravity since these in Newton’s Laws of Motion, and also middle, and high school levels achieve terms are interrelated threshold concepts the relationship between force and mo- Next Generation Science Standards. for several of the performance expecta- tion which is subject to the common tions in the Next Generation Science misconception that gravity does not act Historical Background Standards (NGSS Lead States, 2013). on all objects, however they may be mov- In many cases the trajectory of a As the NGSS becomes widely adopted, ing. For example, many students believe child’s understanding of a concept may students will be expected to have a ro- that gravity does not act on an object that parallel the development of a concept in bust scientifi c understanding of these has been thrown upward, until it reverses the history of science. Consequently, a concepts in order to achieve a number direction and starts to fall (Palmer, 2001). brief historical review can provide use- of performance expectations. Following ful insights for researchers by suggesting is a list of performance expectations that MS - ESS2 - 4. Develop a model the possible causes of learners’ diffi cul- require understanding of weight, mass, to describe the cycling of water ties, as well as for teachers to be aware and/or gravity. through Earth’s systems driven by of possible misconceptions that their 1 energy from the sun and the force of students may hold. Children can also 5-LS1-1. Support an argument that gravity. plants get the materials they need benefi t from accounts of scientists of the for growth chiefl y from air and Traditionally the water cycle has been distant past who may have shared some water. taught at the elementary level. Mov- of their own ideas about the world, and ing it to the middle school will make it how the scientists eventually changed If it were not for the compelling illustra- more accessible, but still challenging as their ideas as a result of new data, or new tion on the video, it might not be obvious it requires students to conserve matter ways of thinking (Bar and Zinn, 1998).