Paper Bridge Materials Challenge

Background Information for Teachers

If you haven't already, read the background section for the first paper bridges lesson plan. In that lesson, students explored how a bridge's shape can affect its strength. In this activity they will explore how a bridge's material affects its strength. What happens if you make the bridge out of aluminum foil, wax paper, or cardstock instead of regular paper (Figure 1)?

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 1. The same bridge design made from four different materials.

Different materials have different properties. In this project, we are specifically concerned with the material's mechanical properties (as opposed to other types of properties, like chemical or optical properties). How easy is the material to stretch, squeeze, tear, or bend? If you bend the material slightly, does it bounce back to its original shape, or stay bent? How far can you stretch or bend the material before it breaks? Does it break slowly (like stretching out a ball of clay) or suddenly (like snapping a wooden pencil)? All of these factors determine what materials engineers use to build bridges. For example, if a bridge flexes a little bit under a large amount of weight (like a train), you want it to return to its original shape.

In this project, first your students will explore the mechanical properties of some different materials. They will use their observations to predict which material will make the strongest bridge. Then they will design an experiment to test bridges made from different materials and see which one is the strongest.

Additional Background Links

• Material Properties, BBC Bitesize
• Bridge Construction & Materials, Harford County MD

Prep Work (5 minutes)

• A standard sheet of printer paper is 8.5 by 11 inches. For this activity to work well, you should have sheets of all the other materials that are about the same size. Cut them to size as needed. For example, a roll of aluminum foil might be 12 inches wide, so you can tear it into 8.5 inch sheets and then trim one inch off the edge.

Engage (5 minutes)

1. Ask students to think about different types of bridges they have seen in real life. You can tailor this discussion to your geographic region (e.g. specific bridges in your town that you think students will be familiar with). Optionally, if you have internet access in your classroom, you could have students search online for pictures of different bridges.

   Are all bridges made from the same materials? Why or why not?

   Not all bridges are made from the same materials. Ancient bridges were made from stone and wood. Modern bridges are made from materials like steel and concrete. Different materials can have different strengths and weaknesses. For example, some materials might be more expensive than others, some might be stronger, and some might be lighter. There are multiple factors that must be considered when deciding the "best" material for a bridge.

2. Remind students that in the previous lesson they built bridges out of paper, and tested how much weight they could hold using pennies. In this lesson they will have three new materials available to them: cardstock, aluminum foil, and wax paper.

   Which material do you think will make the strongest bridge? How could we find out?

   Allow students to guess which material they think might be the strongest. But, just guessing is not good enough—now they will design an experiment to measure which material makes the best bridge.

Explore (30 minutes)

1. Before you start building bridges, explore the mechanical properties of each material as a class.

   Before we build bridges, how could we get an idea about which materials are stronger?

   We could take sheets of each material and do things like bend, stretch, rip, or fold them.

2. Decide, as a class, what tests you will do on the different sheets of material. Plan how you will use the blank data table in the student worksheet to record your results. Remember to let the students come up with the ideas, but you can provide suggestions if they have trouble. For example, you could ask questions like:
   • If we bend the sheet slightly, does it go back to its original shape? (yes/no)
   • How hard is it to roll the sheet into a circle? (rank the four materials relative to each other)
   • How hard is it to fold and crease the sheet? (rank the four materials relative to each other)
   • If we place the sheet across two stacks of books (like in the first paper bridge activity), does it fall under its own weight? (yes/no)
3. Working in pairs, have each group perform the tests and record their results on the student worksheet. Discuss the results as a class.

   Do any of your results surprise you? Did some materials do better than others in certain tests, but not others? Now, knowing what you do about these materials, which one would you pick to build a strong bridge? Why? Is this the same as your original prediction?

4. As a class, decide on a plan to test which material makes the strongest bridge. Let your students come up with the plan and provide guidance as necessary.

   Now that we know a little more about the strength of each material, how could we test them to see which one makes the strongest bridge?

   Each group could build one bridge from each sheet of material and test them just like we did with the paper bridges (adding pennies until they fail). We could then compare results to see which material held the most pennies for each group. Note: it is important, within each group, for the shape of the bridge to remain constant, so they can make a valid comparison between the materials (you can't compare a triangular paper bridge to a square aluminum foil bridge). For example, each group could pick their final design from the first paper bridge activity, and build four copies of that bridge using different materials. It is OK for different groups to use different shapes.

5. Remember to follow these rules when testing the bridges:
   1. Set up two stacks of books 10 inches apart. When testing, place the bridge so one end rests on each stack of books.
   2. You can use up to two pieces of tape for each bridge to prevent it from unfolding/unrolling. You cannot tape the bridge to the books.
   3. Add pennies to the bridge one at a time, starting at one end and working your way to the other side. If you reach the other side, go back to the beginning and stack the pennies on top of each other.
   4. Keep adding pennies until the bridge collapses (falls down and touches the table) or the pennies start to slide/fall off the bridge (Figure 2).
   5. Use the student worksheet to record how many pennies each bridge could hold.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 2. Testing bridges made from different materials. Your students might only have enough pennies to test the bridges one at a time.

6. Ask each pair to look at their own results before comparing data as a class.

   Rank your four bridges from strongest to weakest. How does this ranking compare to what you expected?

   Which bridge held the most pennies? Why do you think it was the strongest?

Reflect (5 minutes)

Now combine the results from each group and discuss your results as a class.

1. Make a data table on the board that you use to record information about the strongest bridge for each group. Ask each group to report their results.

2. Make another table on the board to keep a tally of the strongest materials. Fill this table in based on the information in Table 1. For example, if there are three bridges with "paper" in the Strongest Bridge Material column in Table 1, write "3" for Paper in Table 2. If your students are learning about bar graphs, you can use the data in this table to make a bar graph.
   
   

   Material	Number of Bridges
   Paper
   Cardstock
   Aluminum Foil
   Wax paper

   Table 2. Table to determine which material is the strongest.

   Did every group get the same results (find the same strongest material)? If not, why do you think some groups had different results?

   When we look at data for the entire class, which material was the strongest overall?

   How does this compare to our predictions about what the strongest material would be?

   Why do you think this material makes the strongest bridge? Does this relate to the tests we did with the sheets of material earlier?

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Materials Scientist and Engineer

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Career Profile

Materials scientists study the properties of different materials—not just the mechanical properties, but other types like chemical properties (how will a bridge hold up when it is exposed to different liquids and gases?) or thermal properties (what happens to the bridge when it is really hot or really cold?). They even design new materials that we can use to build bigger, stronger bridges and other structures. Read more

Lesson Plan Variations

• When building a real bridge, engineers must also consider the cost of the materials, not just the strength. Some materials are much more expensive than others, so a cheaper material might be used if it is strong enough. You can easily incorporate math into this lesson by calculating prices for a sheet of each material. For example, how much does a pack of paper cost, and how many sheets are in a pack? Use that information to calculate the cost per sheet for each material. Which type of bridge was the most expensive? Say that you know a bridge will only ever need to support one row of pennies (real cars would not stack on top of each other!). Is it worth it to use a more expensive material, or can you use one of the cheaper materials?
• Engineers do not like to waste material when building real bridges. Using too much material can be more expensive and make the bridge heavier. Let your students use scissors to cut their sheets and remove material. They can trim the sheets to be narrower, or even cut holes in them. Use a kitchen scale with at least 0.1 gram resolution to weigh the sheets. Can they design bridges that are lightweight but also strong? Calculate the strength-to-weight ratio for each bridge (the mass of the pennies it can hold divided by the mass of the bridge). For example, a bridge that has a mass of 2 grams and can hold 20 pennies as a strength-to-weight ratio of 20/2 = 10 pennies per gram.
• Combine the previous two variations to calculate the cost effectiveness of each bridge (how much weight it can hold per amount of money spent on construction materials). For example, say that a whole sheet of printer paper costs 1 cent. You cut enough holes in the paper to reduce its weight by half, so that is 1/2 cents' worth of paper. If that bridge can hold 20 pennies, then its cost effectiveness is 20/0.5 = 40 pennies per cent.