Straws and Pins--Building Out

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What It Is (continued)

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.


Discussing Results

Have the students share their results of free exploration with the whole class. Record their discoveries and questions on the chalkboard or chart paper.

In the challenge activities, issues of winners and losers may arise. Look for various criteria to judge buildings, including the degree of risk, dimension, and stability. The smallest structure may be the most stable, the most innovative may have collapsed early on. Emphasize what lessons were learned from each structure.

During testing with weights, questions like the following can help the students see some of the implications of what they are doing:

  • How many weights can be hung before the structure collapses?

  • Does it matter where the weights are hung?

  • Will the structure be more likely to collapse if the weights are hung in one place or spread out?

  • Predict where you think the structure will weaken first. Can that area be strengthened so that another area will collapse first?

    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.


    What's Going On

    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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