How Strong are Eggshells?
IntroductionHave you ever seen an arch structure in a building, such as over a doorway or surrounding large windows? Arches have been used in structural engineering since ancient times. In this activity you will test the strength of a naturally occurring arch shape: the shell of an egg. So grab some eggs and put them to good engineering use in trying to answer the following question: Just how much mass can an eggshell can support?
- Raw eggs (at least 3)
- A bowl
- Dinner plate or other large, flat surface to place eggshells on for testing
- Pencil or marker
- Optional: Ruler
- A rotary tool, sometimes called a dremel, similar to Amazon.com. Alternatively it is possible, but more challenging, to use a small triangular file.
- Safety goggles, to protect your eyes from bits of egg shell when using the rotary tool
- Hardcover book
- Several magazines or light-weight books
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- Use a pencil or marker to make a line all the way around one of the eggs, dividing the egg halfway between its two pointed ends. This line should approximately be at the eggs' widest point (width-wise, not length-wise). You may want to use a ruler to help you determine the halfway point as you make the line.
- Carefully crack the eggshell at the pointy end. Make a small hole and drain the contents of the egg into a bowl. Rinse the empty eggshell out with some water. Remember to wash your hands carefully when handling eggs, including the shells, as raw eggs can carry Salmonella. Salmonella is a major cause of food poisoning.
- Use a triangular file, or rotary motor tool with a cut-off disk, to score the eggshell on your marked line, all the way around. If you are using a rotary tool with a cut-off disk, work with an adult and wear safety goggles. You may also want to wear a dust mask. Work slowly, using just the edge of the cut-off disk.
- Carefully break or cut the eggshell back to the scored line you created. Slowly break off small pieces of the shell, one at a time, working your way around.How hard does the eggshell feel as you handle it?
- Repeat this process two more times so that you have prepared a total of three eggshells.
- It is okay if the edges of the eggshells are a little jagged, but if any prepared eggshell half develops big chips or hairline cracks, you will want to start over with a fresh egg. There should be no cracks or big chips weakening your prepared eggshells.
- Place the three prepared eggshells on a flat surface, like a dinner plate, with their open end facing down. Space them apart equally on the surface so that they form an equilateral triangle.Why do you think it is important to equally space them apart?
- Carefully lay a hardcover book on top of the three prepared eggshells. The book should be centered over the eggshells so that the mass will be distributed evenly among them. Do the eggshells support the weight of the book?
- One at a time, carefully add magazines (centered on top of the book) to see how much mass the eggshells can support.How many magazines can you add before the eggshells crack and break? Is the final load supported by the eggshells heavier or lighter than you thought it would be?
- Be sure to thoroughly clean any surface the raw eggs touch (including the shells) because they can carry Salmonella.
The eggshells likely supported a surprisingly heavy load. However, if any eggshell had a hairline crack or a big chip, the eggshells may have only held the book and a couple of magazines before the weakened eggshell cracked, was crushed, and then the others were crushed as well. In one trial at Science Buddies we saw them hold up a small hardcover book and a stack of 15 magazines before all of the eggs cracked and were crushed under the load.
In a building, force from the mass of anything that is on top of an arch gets distributed to the piers on the sides of the arch. This redistribution is due to the arch's shape, and is similar for the arch of the eggshells as well. This redistribution gets rid of some of the stress that is being placed on the structure. Overall, the arch's shape allows it to support a relatively large amount of mass being placed on top of it.
Arches have been used in structural engineering since ancient times. For example, over 2000 years ago Romans utilized the arch in aqueducts and other structures. Arches are structures that 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.
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 is 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 will find that it can withstand more force than you might expect. (If you try this, be sure to wear work gloves because the eggshell pieces will be sharp if you break the egg.)
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For Further Exploration
- Use a kitchen scale to measure the combined mass of the book and magazines that the eggshells supported without breaking. Just how much mass could the eggs support? Are you surprised by your results?
- In this activity, 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?
- Try this activity again but cut the eggs length-wise instead of width-wise. How much load-bearing capacity do eggshells have when prepared this way?
- Blog Post: The Science of Bridges
- Blog Post: Building Bridges
- Blog Post: Egghead Science: The Strong and the Weak of It