Spanning the Desk

Here is one of the winning paper bridges that the Principles of Physics classes built this month.

Students were given one piece of 8.5 x 11 paper with which to build a bridge that would span 8 inches and hold as many pennies as possible. They could fold their paper any way they wanted, or cut off strips to use as supports.

Before building, classes tried an interactive website at PBS.org to determine which shapes were strongest. Some students used the triangle shape, which had proven victorious in holding the most imaginary elephants on the interactive site. Others chose the arch or circle, and some used the rectangle shape. One crafty group decided to make a bridge shaped like a coin roll. The only problem was filling it with pennies, since the bridge had to be set into position, then filled. It was hard to poke the pennies into a narrow tunnel.

The overall winner had a flat bottom and arched roof, and slowly sagged its way toward the desktop as it filled with pennies, until finally touching bottom and being declared collapsed.

Only one group chose Mrs. Cureton's favorite style, the accordion fold. The first year of building paper/penny bridges, Mrs. Cureton's bridge trounced all competitors by over a hundred pennies. Last year she was soundly beaten by a student using her favorite design.

You will notice that the pictured bridge utilized the triangle shape on the sides for support. The manner in which this bridge failed was the lack of sides to hold in the pennies. One little error in penny placement, and a cascade of pennies leaked over the side, causing the bridge to crease from tension on the paper, and then twist from the torsion force.

With the accordion design, you fold a piece of paper back and forth like a fan. Start putting pennies along the creases, from each end. Once the ends near the supports are full, start working towards the middle, again making sure to apply pennies alternately from both ends. You can see that an accordion design has much more surface area on which to place the pennies than the flat style shown above.

Give it a try, and see if you can beat 247 pennies before collapse.