Have you ever noticed that some big doorways—especially in old buildings—have arches over them? Arches have been used in engineering since ancient times. In this activity you'll test the strength of a naturally occurring arch shape: the shell of an egg. So grab some leftover Easter eggs and put them to good use in trying to answer the following question: Just how much mass can an eggshell support?
Arches have been used as important elements in structural engineering for thousands of years. For example, more 2,000 years ago Romans used the arch to give support for aqueducts and other large stone structures. The shape of the arch distributes the 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'll 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 do break the egg.)
- At least three raw eggs
- Pencil or marker
- Ruler (optional):
- One bowl
- Small triangular file or a rotary motor tool with a cut-off disk (If you are using a rotary tool, you will also need adult help, safety goggles and may want to wear a dust mask.)
- Dinner plate or other large, flat surface to place eggshells on for testing
- Hardcover book
- Several magazines or lightweight books
- 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 (around the egg width-wise, not lengthwise). 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. Be sure to thoroughly clean any surface the raw eggs touch (including the shells and your hands) with soap and water because they can carry salmonella.
- 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. Why do you think it's important to work slowly on this step?
- 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.
- Note that 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, such as a dinner plate, with the open end facing down. Equally space them apart on the surface so that they form an equilateral triangle. Why do you think it's 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?
- Lift up the magazines and book that could be supported. Are they very heavy? Are you surprised by how much mass the empty eggshells could support?
- Extra: Use a kitchen scale to measure the combined weight of the book and magazines that the eggshells supported without breaking. Just how much weight (mass) could the eggs support? Are you surprised by your results?
- Extra: 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?
- Extra: Try this activity again but cut the eggs lengthwise instead of width-wise. How much load-bearing capacity do eggshells have when prepared this way?