You and Your Students!
Vicki Cobb, Education World Science Editor
Balance a sheet of paper on its end! Stand on a soda can without crushing it!
Technology, Materials Science (Engineering), Physical Science,
Setting the Scene (Background)
In order to build anything, engineers must know how strong a material is and how to shape it to maximize its strength. In this lesson, I offer two amazing activities that will help demonstrate for students how three-dimensional shape strengthens paper and aluminum cans.
You might do these activities as demonstrations first; but be prepared for your students to want to try them on their own.
Before you start the lesson, you might point out to students that the steel beams that are used to build buildings are often shaped like the letter "I." Students might think that solid beams would be even stronger than the I-shaped beams, but that is not necessarily the case.
Act I: Balance a Sheet of Newspaper on Its Edge
Crease a sheet of newspaper across its diagonal. Fold back each side about an inch from the crease to form two "wings." [See picture.] The accordion fold diagonally across the center of the sheet acts as a rib. Now the newspaper will stand up on edge when you balance it on your hand.
Folding materials often makes them stronger by deflecting and diverting forces. Corrugated (or folded) paper, for example, is used to make boxes stronger. (You might have students "dissect" a cardboard box to see how it is constructed from paper.) In a beehive, the hexagonal shape of a honeycomb cell gives the wax a structural strength not possessed by a flat sheet. Long ago, engineers discovered that a beam with the shape of the letter "I" is as strong as a beam the same width/height/length of solid steel; that space between the top and bottom cross bars need not be filled in with steel. Plus the I-beam weighs a lot less and uses a lot less steel than the solid beam of the same dimensions would.
Act II: Another Balancing Act
A tiny dent can destroy the strength of a soda can. The thin wall of an empty aluminum soda can is strong enough to support your weight -- if you weigh less that 250 pounds. However, it's strong enough only if your weight is evenly distributed on top of the can and the can is perfectly free of dents.
For this activity, you might want to be the demonstrator since an adult is a more spectacular subject! Otherwise, select the biggest kid in your class. Getting up on the can is a bit tricky. Have the balancer lean on two other people on either side as he or she places place one foot on top of the can. Make sure that the center of the foot is directly over the can. Gently, still with the support of two others, the balancer should lift the other foot off the ground, slowly transferring weight to the can When the students supporting the balancer take away their support, the can will support the balancer.
When the student is balanced atop the can, poke the can with the eraser end of a pencil. The can will instantly collapse! Talk about that quick comedown.
An aluminum can with walls no thicker than two pieces of paper, can support eight thousand times its own weight! It is essential, however, that the load be evenly distributed directly over the center of the can. The weight may be heavy enough to cause the sides of the can to bulge, but the can won't collapse so long as the weight on it is evenly distributed.
A dent in the can creates a weak spot. Then the weight from above pushes the dent farther out of line and the can collapses.
If you're lucky, the can becomes a nice flat pancake ready for recycling!
An egg is another structure that is extremely strong so long as the force on it is evenly distributed. You can squeeze a raw egg in one hand without breaking it. But the force of a knife on one side easily opens it.
Article By Vicki Cobb
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