Children enter the classroom already knowing something about weight. The fact that objects differ in weight is salient to them, as their sensory systems respond to the pressure of objects on their hands and how much effort they have to exert to hold things or to make them move. How heavy something is not only has consequences for how easy or hard it is to pick up, but also for how it behaves with other things (e.g., light things might be blown away by the wind, whereas heavy things stay put), or how potentially dangerous it might be (e.g., heavy things cause more damage if dropped on one’s foot). Further, children need to be able to unconsciously predict the weight of things to interact with them effectively. When they reach to pick up objects, they have to adjust their muscles for the expected weight of the object. If they are wrong, their hand may suddenly drop down or fly up, which in fact rarely happens. Thus, weight is not only important to scientists, but also to young children. In light of its importance and salience, it is not surprising that “heavy” and “light” are words that children learn very early.
At the same time, children’s initial understanding of weight is limited, and very different from the understanding of scientists, as it is grounded in their perceptual experiences of weight rather than objective comparison and deeper theoretical understanding. The core of weight for children is “felt weight”—how heavy or light something feels. For example, they initially just have a few categories of weight (very heavy, heavy, light) with some small light things judged to weigh nothing at all. They also judge that two visibly different size clay balls weigh the same, because they both feel light, rather than reasoning they must weigh different amounts because they have different amounts of stuff. Weight is not yet a “conserved” quantity: they think that the weight of a clay ball changes when it is flattened into a pancake, or cut into many little pieces, because these transformations affect how heavy the object feels, rather than thinking the weight is unchanged because no clay has been added or removed. Finally, their reliance on “felt weight” encourages the conflation of weight and density. Small dense objects feel heavier than they really are because how heavy something feels depends on weight, density, perceived volume, and other variables.
The challenge is that students must use their initial concept of weight to make sense of new learning (e.g., seeing the similarity between using their two hands and the two pans of the balance scales to compare the weights of objects, forming explicit generalizations such as “big things tend to be heavy” or “plastic things tend to be light”). Then they must use the new learning to restructure their concept of weight (e.g., seeing the balance scale as a better, more reliable indicator of weight than their hands, using the balance scale to see that even small additions of material changes the weight of objects, using balance scales to measure weight in terms of some unit, thus redefining weight as an invariant property of the object that has an exact numeric value—this object weighs 40 paper clips—one that remains the same across transformations in which no material has been added or removed). In the process, what was initially their bedrock—their reliance on felt weight—will be called into question and what was initially less familiar or completely unknown—relations between weight and amount of matter, use of a variety of measurement tools that depend on numerical reasoning—will become their new bedrock. This kind of learning does not occur overnight or without serious reflection on and discussion of new first hand experiences—for example predicting weight orders by felt weight and then confirming or negating this order using the balance scale, asking “what if questions” to oneself, discussing reasons for observed discrepancies. This strand provides students with experiences that help them begin to make this transition, as they discover the strengths and limits of felt weight, link weight to the amount of material in an object, and learn to use the balance scale, first to compare, and then to measure weight.