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• Science Fair Project Guide

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• Stress, Strain and Strength

Project Summary

Difficulty	5
Time required	Short (several days)
Prerequisites	None
Material Availability	Readily available
Cost	Low ($20 - $50)
Safety	Adult supervision required if motor tool is used.

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Abstract

Objective

The goal of this project is to measure the weight-bearing capacity of eggshells.

Introduction

Arches have been used in structural engineering since ancient times. The figure below shows a Roman aqueduct (Pont du Gard, France), built in about 19 B.C. Arches allow passage through a structure, for example: light through arched windows, or people through arched doorways, or water passing under arched bridges. The shape of the arch distributes the compressive forces to the weight-bearing piers that support the arch.

Figure 1. Pont du Gard, France, a Roman aqueduct built about 19 B.C.

An eggshell is a natural example of an arch. One end of the shell has a larger, rounder arch, and the other end is narrower and more pointed. It's pretty easy to crack an eggshell if you tap it against a hard surface. But if you interlock your fingers and try to squeeze an egg lengthwise to break it, you'll find that it can withstand more force than you might expect. (You might want to wear work gloves for this test, because the eggshell pieces will be sharp if you break the egg.)

In this experiment, you will measure the weight-bearing capacity of eggshell arches. Before starting on your experiment, you should do background research on arches. Learn about different types of arches, and how strength changes with the shape of the arch. You should also do some background research on eggs to find out what material the shell is made from. After you've finished your background research, make a prediction about how much weight you think an eggshell can support. Then do your experiment and find out for yourself!

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Arch
• Compression
• Tension
• Eggshell

Questions

• Of what materials are eggshells made?

Bibliography

• Here are some webpages with information on arches and their use as structural supports:
  • The Science Museum, 2004. "Making the Modern World: The Arch Bridge," The Science Museum, U.K. [accessed April 5, 2007] http://www.makingthemodernworld.org.uk/learning_modules/maths/02.TU.03/?section=4.
  • Wikipedia contributors, 2007. "Arch," Wikipedia, The Free Encyclopedia [accessed April 5, 2007] http://en.wikipedia.org/w/index.php?title=Arch&oldid=120563647.
  • Morissey, M., 1998–2007. "How Bridges Work: The Arch Bridge," HowStuffWorks.com [accessed April 5, 2007] http://science.howstuffworks.com/bridge5.htm.
• For information on chicken eggs, see:
  Enchanted Learning, 2003. "Chicken Egg Development," EnchantedLearning.com [accessed April 5, 2007] http://www.enchantedlearning.com/subjects/birds/info/chicken/egg.shtml.
• You can find more information on materials testing in the Science Buddies resource Stress, Strain and Strength.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• Eggs (at least a dozen)
• Bowl (for egg contents)
• Small triangular file
  • You should be able to find this at your local hardware or hobby store, most likely as part of a set of small files.
  • An alternative is to use a rotary motor tool. (Dremel is the major brand name. A basic set can be purchased for under $40. http://www.amazon.com/Dremel-275-02-MultiPro-Rotary-Accessories/dp/B0000302XU/
    ref=sr_1_3/104-6984742-5113504?ie=UTF8&s=hi&qid=1175883113&sr=1-3) with a cut-off disk. Wear safety goggles! Using a dust mask is a good idea.
• Ruler
• Pencil or marker
• Weights, e.g.:
  • One hardcover book (for first layer)
  • Stack of magazines (to be added one at a time for remaining layers)
• Kitchen scale (to weigh book and magazines)

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Experimental Procedure

1. For each trial you will need three eggshells, prepared in the same way.
   1. Carefully crack the eggshell at the pointy end.
   2. Make a hole and drain the contents of the egg into a bowl. (You can use the egg contents for cooking.)
   3. Carefully break or cut each eggshell back to the same height. In the image below, the drained eggshell is being measured for cutting. In this case, the egg will be cut approximately at its widest point (blue line).
      • Mark a line all the way around the eggshell to use as a guide.
      • If doing by hand, use the triangular file to score the eggshell on your marked line, all the way around.
      • Carefully break off small pieces of the shell, working your way around.
      • This part is tricky, so take your time.
      • The edge will be a little jagged, that's OK.
      • As an alternative to a file and breaking by hand, you can use a Dremel motor tool with a cut-off disk. Wear safety goggles! Work slowly, using just the edge of the cut-off disk. Follow your marked line carefully.
      • If the eggshell breaks badly, start over with a fresh egg. There should be no large cracks weakening your prepared eggshells.

2. Place the prepared eggshells on a flat surface, with their open end facing down. The distance between each of the eggshells should be equal (i.e., the eggshells should form an equilateral triangle).
3. Carefully lay the book on top of the three eggshells, as shown in the diagram below. The book should be centered over the eggshells, so that the weight will be distributed evenly among them.

4. Carefully add magazines, one at a time, to see how much weight the eggshells can support.
5. Use the kitchen scale to measure the weight of book and magazines that the eggshells supported without breaking.
6. Repeat with at least three sets of eggshells.
7. Calculate the average weight supported, per eggshell.
8. More advanced students should also calculate the standard deviation to see how much variability there was in the results.

Variations

• In this experiment, you broke the pointy end of the egg, and measured the strength of the arch made from the larger diameter curve. What do you think the results would be if you instead broke the larger end of the egg, and tested the strength of the "pointy" arch?
• Legend has it that the dome of Mormon Tabernacle in Salt Lake City, Utah, was based on the shape of an eggshell, cut in half parallel to its long axis. How much weight-bearing capacity to eggshells have when prepared this way?
• Here's another (slightly messier) way to measure the strength of eggshells using whole eggs and a simple experimental apparatus:
  Fish, F.E., n.d. "An Egg-ceptionally Egg-celent Egg-periment: Testing the Influence of Morphological Design," Department of Biology, West Chester University [accessed June 14, 2007] http://www.sicb.org/dl/biomechanicsdetails.php3?id=30.

• For more science project ideas in this area of science, see Materials Science Project Ideas.

Credits

Andrew Olson, Ph.D., Science Buddies

Sources

• This idea is from an entry to the 2007 San Mateo County Science Fair (author's name not provided).

Last edit date: 2007-06-14 21:00:00

Career Focus

If you like this project, you might enjoy exploring careers in Materials Science.

Industrial Engineer You’ve probably heard the expression “build a better mousetrap.” Industrial engineers are the people who figure out how to do things better. They find ways that are smarter, faster, safer, and easier, so that companies become more efficient, productive, and profitable, and employees have work environments that are safer and more rewarding. You might think from their name that industrial engineers just work for big manufacturing companies, but they are employed in a wide range of industries, including the service, entertainment, shipping, and healthcare fields. For example, nobody likes to wait in a long line to get on a roller coaster ride, or to get admitted to the hospital. Industrial engineers tell companies how to shorten these processes. They try to make life and products better—finding ways to do more with less is their motto.

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