How to Build a Paper Bridge
1Bridges carry people and cars over rivers, roads, and deep valleys. Engineers must be sure that a bridge is strong enough to hold heavy loads. In this activity, you will test two paper bridges to find out whether the shape of a material can change how strong it is. Read each step carefully, because the steps must be done in order.
2Materials: You will need two stacks of books that are exactly the same height, two sheets of paper, a small paper cup, and a handful of coins. Keep the stacks the same height so that each paper bridge lies level.
3Step 1: Set the two stacks of books on a table. Leave a gap between them that is a little shorter than a sheet of paper is long. Lay one flat sheet of paper across the gap so that each end rests on a stack.
4Step 2: Place the paper cup in the middle of the flat sheet. Add coins to the cup one at a time. Stop when the paper sags, or bends down in the middle, so far that the cup tips over. Count the coins that were in the cup, and write that number down.
5Step 3: Take the second sheet and fold it back and forth in narrow accordion folds, about as wide as your finger, with the folds running the long way down the sheet. Lay the folded sheet across the same gap. Use the same gap and the same cup each time, so that the test stays fair.
6Step 4: Place the cup in the middle of the folded sheet and add coins one at a time, just as you did before. Stop when the cup tips over. Count the coins and write that number down.
7Step 5: Compare your two numbers. Most of the time, the folded sheet holds many more coins than the flat sheet.
8Why does folding help? A flat sheet bends easily in the middle because nothing holds it up. The folds make a row of ridges. Each ridge helps hold up the ridges beside it and spreads the weight of the cup across the whole sheet. Builders use the same idea when they bend metal into ridged roof panels. They also test their ideas carefully before they build, just as you did with your coins.