During the building process, the
students should be encouraged to test the strength, stability, and durability
of the structures they are working on. At the end of the building process,
students should have the opportunity to observe and discuss each other's
structures. For challenges involving strength, hang weights to test the
strength of each structure one by one, so that the students as a class can
observe what works and what doesn't work.
In discussing any structure, special attention should be paid to structural elements that worked well or that the students learned to avoid. Attention should be directed to how individual structures change from near the support points to far from the support points.
What kind of problems did the students run into and how did they solve them? Often when building out, especially with cantilevers, the problem of twisting to one side or the other arises. Look for this problem and ask students how they dealt with it.
Look for tension and compression elements in these structures. Often, a long line of straws at the top of a structure will be in tension and a long line of straws along the bottom will be in compression.
If you were hanging from a tree branch with a hungry bear below you, would you be better off near the tree trunk or at the end of the branch? From these experiments, you probably know that if you're dangling from the end of the branch, the branch is more likely to droop and possibly break off, leaving you at the mercy of the bear. You can see this in your experiments with straws. Specifically, if you pin a bundle of ten straws to a single straw and hang it off the edge of a table, you will see that the ten straws will be supported. However, if you pin those ten straws end-to-end to your single straw and hang it off the edge of a table, the ten straws will droop considerably. The weight of the ten straws has not changed. But considered as a whole, the weight of the straws is farther away from the supporting edge of the table. Another way of saying this is that the center-of-mass has moved farther from the table's edge. This combination of weight and distance is called torque.
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