In this investigation, 8th grade students conducted an experiment to explore a linear relationship as they tested how bridge thickness is related to strength. They displayed their collected data in a table and a graph, looked for relationships, and used the relationships to make predictions.
As I launched the investigation, we brainstormed ideas as to what factors affect how strong bridges are and what they notice about the construction and design of them. What materials are used and if they think the relationship of bridge strength and thickness is linear. We used our experiences of how we felt when crossing bridges to make the connection relevant to the investigation. We reviewed what the dependent (number of pennies, breaking weight) and independent (bridge thickness) variables of the experiment are. We also discussed the things that needed to be constant throughout the entire experiment, such as how (with what type of force) the pennies are placed into the cup, making sure the bridge is overlapping the books by 1 inch, and making sure to discard the bridges that were used as to not compromise the experiment.
Two books of the same thickness
A small paper cup
About 50 pennies
Several 11-inch-by-4.5-inch strips of paper folded up 1 inch on each long side
Students worked in groups of 4 with 7 groups in the classroom. They had to start with one of the paper strips and suspend the bridge between the books making sure it overlapped each book by 1 inch. They placed the cup in the center of the bridge and put pennies into the cup, one at a time, until the bridge collapsed. Students recorded the number of pennies they added to the cup and recorded the breaking weight. Students then put two new strips of paper together to make a bridge of double thickness, repeated the process, and found the breaking weight for this. They repeated the experiment to find the breaking weights of bridges made from three, four, and five strips of paper. Students made a graph for their data and discussed in their groups the relationship between bridge thickness and breaking weight. They discussed if there appeared to be a linear or nonlinear relationship and described how it was shown in table and graph. Students then made predictions of breaking weights for a bridge 2.5 and 6 layers thick and explained their reasoning.
Students’ ideas were shared with the whole class and a large graph was displayed at the front of the class where each group plotted their points to see the data as a class. Students saw the relationship as being linear not only in their groups, but also as a whole class.
Students were engaged in the experiment because of its kinesthetic approach and connections made with content and prior knowledge of linear relationships and their experiences which got the students to wonder, question, and discuss the engineering behind building bridges.
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