Key Concepts
Physics
Engineering
Materials
Force
Strength
Introduction
If you've looked at bridges, you've probably noticed that they can be made of different materials. Some are wood, some are steel, some are concrete, and some are even made from stone. If you want to build a simple, sturdy miniature bridge using something you have lying around the house, what's the best material to use? Try this activity to find out!
Background
What materials should engineers use to build a bridge? The decision depends on many factors. Where is the bridge located? How long will it be? What will go across the bridge (people, cars or trains) and how heavy will the total load be?
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You probably know that different materials have different properties. You might think about the properties of various materials using your senses—for example, how something looks or feels; is it light or dark; smooth or rough? There are also properties that describe how strong a material is. These are called mechanical properties. For example, how hard is a material to stretch, squish or bend? When you bend the material and then let go does it stay bent or go back to its original shape? If you bend the material so far that it breaks, does it happen very slowly or snap suddenly and unexpectedly?
You might have experienced these different properties in everyday life without realizing it. When you bend a paper clip it stays bent; it's also hard to make the paper clip "snap" suddenly. Compare that with a wooden ruler. If you were to flex the ruler a little bit, it will bounce back to its original shape; but if you were to bend it too far, it would snap. Some materials, such as rubber, are easy to stretch or squish. Other materials, such as rocks, are much stiffer. You can probably imagine what properties would be important for a bridge. If a heavy truck drives over a bridge, do you want the bridge to sag a lot? Do you want the bridge to return to its original shape after the truck leaves? In this activity you'll explore these properties with various household materials and decide which one would make the best bridge.
Materials
Two thick books or small boxes
Paper
Wax paper
Aluminum foil
Scissors
Tape
At least 25 pennies. (If you don't have enough pennies, you can use fewer heavier coins, such as quarters, or other small objects to use as weights, such as small rocks, bolts, and so forth.)
Ruler or measuring tape (optional)
Preparation
Cut your wax paper and aluminum foil into sheets the same size as the piece of paper.
Space your books roughly 10 inches apart.
Fold each of your three sheets of material into a bridge shape: First fold them in half lengthwise at least twice. Then fold up the edges to make walls. Tape the edges to prevent the bridges from unfolding. Make sure your bridge is wide enough to hold pennies sitting flat. Which material do you think will make the sturdiest bridge?
Procedure
Rest one of your bridges across the gap between the books.
Place a penny in the middle of the bridge.
Keep adding more pennies to the bridge one at a time. Space the pennies evenly along the length of the bridge. This simulates how real people or cars travel across a bridge; they aren't all piled on top of one another.
Watch the bridge carefully as you keep adding pennies. Does the bridge hold its shape or start to sag?
If you fill the entire surface of the bridge with pennies, start a second layer. Keep adding pennies until the bridge collapses. How many pennies did it take to make the bridge collapse?
Repeat the procedure for each of your other bridge materials. Which bridge held the most pennies? Did the bridges all collapse in the same way (i.e. did some collapse suddenly and did some collapse gradually)?
Extra: Build a fresh bridge from each type of material. Add some pennies to each bridge—but not enough to make it collapse—and then remove the pennies. Does the bridge completely return to its original shape—or is it permanently deformed?
Extra: The shape of an object also affects its strength. Try changing the geometry of your bridges, adjusting, for example, the width of the bottom or the height of the walls or the number of times you fold each sheet in half. How does changing the shape affect how many pennies the bridge can hold?
Observations and Results
You probably found that paper made the strongest bridge. You might have been surprised to find out that the aluminum foil bridge wasn't the strongest. After all, isn't metal stronger than paper? An object's strength depends not just on its material but also on its dimensions. A thick piece of paper can be harder to bend than a thin piece of metal—and aluminum foil is very thin.
If you watched closely, you also might have noticed that the bridges did not all collapse in the same way. The paper bridge might have started to sag gradually, eventually falling and dropping the coins. The aluminum foil and wax paper bridges might have failed much more suddenly—mostly holding their original shapes and then rapidly collapsing. How would you take this information into account when building a bridge of your own?
More to Explore Paper Bridges, from Scientific American Suspension Science: How Do Bridge Designs Compare?, from Scientific American Strength in Numbers: Spaghetti Beams, from Scientific American STEM Activities for Kids, from Science Buddies
This activity brought to you in partnership with Science Buddies
