Time 10–15 minutes
Materials
(per group of two)
• 7 straws
• 14 paper clips
Alternate Materials
Toothpicks and dried peas or white beans, soaked overnight before the activity
Video Connection
After the activity, show "Geodesic Domes" from Domes or "Trusses" from Bridges show the importance of triangles in structures. (Check Program Descriptions to locate the show segments listed above.)
Try the Shapes Lab
Icebreaker
Give kids the straws and paper clips. Have them brainstorm adjectives that describe the straws. Ask: How do the straws bend? How useful do you think straws are as a building material? (not very useful) How might the straws' qualities change depending on how they are used? (Kids may suggest reinforcing the straws or changing their shape.)
Lead the Activity
•Demonstrate how to connect straws with paper clips, as shown on the Straw Shapes Activity Handout. Push a paper clip inside one end of a straw. Link another clip to the first clip and push the second clip into the end of a second straw.
• As groups finish building their shapes, talk about their plans for testing them. Will one person do the testing or will they take turns? How can they make the test as "fair" as possible? (The same person, or both people, should test both structures.)
The Big Idea
Straws arranged into triangles form more stable shapes than straws arranged into squares. When compression force is applied to the joints, a triangle changes shape less than a square. When compression is applied to a square, as shown below, the joints rotate easily, and the shape changes. In a triangle, the compression in the two sides is balanced by the tension in the cross-piece at the bottom, which pulls the sides back together. This balancing of forces results in a more stable structural form.
To reinforce the importance of triangles, take kids on a "shape" scavenger hunt, through photographs or during their daily travels, looking for examples of shapes used in structures. Scaffolding cross-braces and trusses under bridges and railroad overpasses are good places to see triangles.
Build on It
• Ask kids how they could add two straws to make a four-sided straw square stronger. Possible outcome: Use a diagonal cross-piece made from two straws as a triangular brace. The triangle shape stabilizes the joints of the square and keeps them from changing shape when a force is applied. This is the prinicple behind trusses.
• Encourage kids to incorporate triangles into a three-dimensional structure. Possible outcome: Kids may find that a tetrahedron (three triangular sides with a triangular base) is very strong.
Make Connections
Math Discuss the different two-dimensional straw shapes as polygons. As a group, make a table listing names of polygons and how many sides each has, and have kids build each polygon out of straws. As the number of sides increases, do the polygons become more or less stable?