The Exploratorium Guide to Scale and Structure From the book, The Exploratorium Guide to Scale and Structure Published by Heinemann.






TIME

· 45 minutes for exploration

· At least 1 hour for challenge

PREPARATION

· Separate the materials into packets for each pair.

GROUPING

· Students may work individually or together in pairs. If students have not had much experience building with a partner, this can pose a challenge. You may want to model appropriate behavior.

MATERIALS

(per pair)

· 50 plastic straws

· 50 straight pins or paper clips

· Scissors, for cutting straws

· 10 weights (e.g., large washers or film canisters filled with sand)

· Yardstick or meterstick (can be shared by groups)

TEACHER TIPS

· Use pins only if you think it's safe. Talk about pin safety and rules for working with pins. Rules might include no traveling with the pins, or that pins must stay on the tables or in the straws. Warn students about pricking their fingers.

· Have students take the paper off straws the day before—they'll then be curious about the upcoming activity.

Straws and Pins--Building Up

students with straws and pins

Context

Building up with straws and pins provides students the opportunity to experience many of the basic physics and engineering concepts inherent in structures. Tall structures emphasize concepts such as balancing the forces of weight and the strength of materials as well as stability, tension, and compression. Ask students about the tall structures that they know. Not only monuments, but basketball poles and trees, will come to mind. Discussion of their skyscrapers and out-of-school familiarity with built-up structures will help them connect what they learn in this activity with what they know outside the classroom. You can do the activity early in a unit, and then try variations at a later time.

What It Is

As with any new material, it is important to give your students time to freely explore with the straws and pins. (Some teachers prefer to use straws and paper clips to avoid problems of getting stuck by pins, especially with younger students.) To push students in the direction of building up, ask your students to use 50 straws and 50 pins to build something taller than it is wide. Allow the students to explore their ideas. As their structures begin to take shape, suggest that they use bent paper clips as hangers for weights to test the physical strength of the structures.

After the free exploration, perhaps at the second session, pose one or more of the following challenges using 50 straws and 50 pins:

· What is the highest structure you can build?

· Can you build a 3-foot tower that will hold three sand-filled canisters?

· Build the strongest 2-foot tower that you can.

· What is the highest structure that you can build that will hold one sand-filled canister?

· What are the fewest straws that you can use to build a 3-foot tower?

The question of rules and what is fair inevitably arises in these challenges. This is an important question, as it forms the basis for scientific objectivity. You and the class must define your parameters: Is it fair to anchor the tower legs to the table or floor? Can your tower lean against the wall? Where is it fair to hang the weights? For instance, for the sake of fair comparisons, you and your students may decide that to test the strong 2-foot tower, all weights must hang from a single bent paper clip hooked onto a point which is at least 2 feet high. In setting this condition, students are recognizing that both weight and position (where you hang the weight) are variables in this experiment. In order to get "fair" results, you must change only one variable (the weight) and keep the other variable (where you hang it) unchanged.

Throughout 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, test each structure one by one, so that the students as a class can learn from the efforts of their peers.

Discussing Results

Have the students share their results of free exploration with the whole class. Record their discoveries on the chalkboard or chart paper. Some towers may not be successful. This presents an opportunity to discuss how you often learn more by experimenting with something new, even when it doesn't work, than from something that is true but tried. Underscore the importance of this for learning science.

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, raising 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 if they are spread out?

· Predict where the structure will weaken first. How might 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. Students should be asked what made it work or not work well. For example, triangular sections and diagonal bracing are essential in straw-and-pin structures.

Questions about which straws are being pulled (i.e., can be replaced by strings) and which are being pushed help bring out the concepts of tension and compression.

What's Going On?

When constructing with straws and pins, many physics and engineering concepts come into play to ensure or defeat stability. Your class can directly observe

· tension and compression

· the use of triangular versus square elements and the need for diagonal bracing

· the introduction of broad bases

· the effects of weight distribution

· balance.

All structural action consists of tension and/or compression. In these structures, this means that every straw is either in tension or compression. It is interesting to go through a few structures with your students to try to figure out which straws are in tension and which are in compression. You can determine this by pulling out a pin on one end of the straw. If the two points where the ends of the straw had been attached move apart, the straw was under tension. If the two points move toward each other, the straw was under compression. Your students will readily volunteer that "triangles are strong," and you will most likely see examples of the use of diagonal bracing to stabilize square sections.

tension illustration
compression illustration

You can also ask students where they have seen these shapes in real-world structures. You find them in bridges, broadcast towers, and pylons supporting high-voltage electrical wires. Diagonal bracing can be seen in the structures supporting wooden decks. This engineering principle is quite evident when you work with these materials.

Many of your students' towers will collapse by toppling over to one side. They will find that broader bases give them more stability. Also, hanging weights high on a tower is more likely to topple the tower than hanging weights down low. When the balance point of a tower (its center-of-mass) is not directly over the base, the tower topples over. Your students will get lots of experience with this.

Pin joints are common in real structures. For instance, rivets are used with steel beams and nails are used with wood planks in the same way that pins are used with straws in this activity.

Extensions

· Students should be encouraged to test the strength, stability, and durability of their structures. In this way, they gain insight into the behavior of structures that are subjected to outside forces, and they can see how to improve their structures so as to strengthen weak points. In addition to hanging weights on structures, they can be "wind tested" using a large piece of cardboard as a fan or by taking them outside into a real wind. You can also leave the structures intact overnight, to discover which ones are still standing in the morning.

· Conduct a search for unnecessary straws. Ask the builders if they think there is a straw that could be cut that would not make the whole thing fall down. Have them try it. If it works, could they cut another straw? How many cuts do they think it will take before the structure collapses? Could they build a structure that could not stand many cuts?

· Have the students take a set of materials home to build a structure with their family. They can draw or photograph their structure and write about what they did.

Return to What's New in the World, October 1996.

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