What You Need:
- Two wooden blocks
- A sheet of paper
- A number of coins
To Do and Observe:
1. Set the blocks about 15 cm (6 inches) apart on a table or floor.
2. Create a paper bridge that spans the blocks and supports as many coin "cars" as possible. Fold the paper to make it stronger. You can only stack coins in the center of the roadway, not on top of the blocks. Try folding the paper in different ways to construct the strongest roadway. What other household items will your roadway hold?
3. Discuss your results and determine which structure was the strongest and why.
What's Going On:
Folding the paper as if to make a paper fan makes a corrugated roadway. The folds form a series of triangles. Triangles form the strongest structures.
A structure is anything that supports its own weight against gravity, plus the weight of another object. Weight provides two kinds of force: compression and tension. Compression is a push down on the structure that must be channeled to the ground. Tension is a pull that stretches the structure. Triangles support both of these forces. Compression pushes down equally on two sides of the triangle, causing the base to be pulled equally in two directions, which creates tension. The triangle is the strongest structure because all three sides bear the load. In a square, only two sides of the four bear the load. This equaling of the forces makes the triangle the strongest structure.
The corrugated roadway utilizes this strength to increase the strength of bridges.
Try the same experiment, but alter the distance between blocks. Try different materials for your bridge. What works best?
In order to understand the tension and compression forces that are placed on a structure, create a "people arch" with your children. Two people stand facing each other, at least two feet apart. (You might kneel to become the same height as your child). Both lean toward each other, putting their hands on the other's shoulders. Where do they feel a push (compression)? Where do they feel a pull (tension)?
Challenge your children to try the above experiment using the thickness of two or three sheets of paper. This will dramatically increase the strength of their structure, and enable greater load bearing.