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Unsqueezable Egg

Eggs are similar in shape to architectural domes, which are among the strongest architectural forms.

Arches are curved structures with no angles and no corners. Domes are the three-dimensional equivalent, capable of enclosing a large amount of space without the help of a single column.

When a load is placed on top of an arch or dome, its force spreads out and down the sides of the arch. At the ground, strong supports, or abutments, keep the arch from moving outward. Architecturally, the dome is one of the strongest designs because it supports the weight of the roof evenly so that no single point on the dome supports the whole load and gives way under stress.

Similarly, the arch shape at each end of the egg distributes all the weight evenly and minimizes stress and strain. The egg is strongest at the top and the bottom (or at the highest point of the arch), which is why it does not break when pressure is added to both ends. The curved form of the shell also distributes pressure evenly all over the shell rather than concentrating it at any one point. By completely surrounding the egg with your hand, the pressure you apply by squeezing is distributed evenly all over the egg.

Eggs, however, do not stand up well to uneven forces. This explains why they crack easily on the side of a bowl. Wearing a ring will cause uneven pressure on the egg, cracking it at the point of contact.

Although a hen can sit on an egg and not break it, a tiny little chick can break through the eggshell. The weight of the hen is evenly distributed over the egg, while the pecking of the chick is an uneven force directed at just one spot on the egg.

Examples of architectural arches and domes: the Arc de Triomphe in Paris, bridges over the River Thames in London, and Science World's dome.

Objectives

  • Relate the strength of an egg to its structure.

  • Describe the distribution of weight in an arch or dome.

Materials

  • Per Class or Group:
    2 raw eggs
    2 bowls

Key Questions

  • Why does an egg not break when the hen incubates it but does when a little chick pecks at it from the inside?
  • Would you describe an egg as fragile or strong?
  • In which direction is the force of pressure on the egg in each challenge?
  • Which part of the egg is the strongest? Which is the weakest? Why?
  • Why did the teacher choose volunteers who were not wearing rings? How would wearing a ring have affected the demonstration?

What To Do

  1. Draw 3 eggs on the blackboard.
  2. Invite 2 volunteers to come to the front of the class (make sure they are not wearing rings).
  3. Both of the students hold out his/her stronger hand over a bowl. Place an egg in the palm of each hand.
  4. Challenge them to break the egg by holding the egg between their thumb and forefinger and squeezing the top and bottom of the egg. Rules: no swinging, no smacking, no using other objects, no using any other hands.
  5. Indicate the directions of force on the first egg drawing.
  6. Each student will close his hand so that his fingers are completely wrapped around the egg.
  7. Challenge them to break the egg by squeezing it with even pressure all around the egg.
  8. Indicate the directions of force on the second egg drawing.
  9. Challenge the students to press only one side of the egg. Ask the students to wash their hands.
  10. Indicate the directions of force on the third egg drawing.

Alternate ending:

When your students are unable to break the egg a dramatic end is to snatch the egg from them arrogantly proclaiming that they must be too weak, then try to squeeze the egg yourself. To your students amusement you will also be unable to break the egg. Feigning embarrassment you can then proclaim that you forgot to use your stronger hand. Swap the egg to your other hand (while wearing a ring, the best position is mid-finger) and proceed to easily crush the egg. Ask students to figure out if your other hand is really that strong or if some other factor allowed you to crush the egg.

Extensions

  • Test various kinds of eggs: chicken, quail, duck — does the size affect its strength? How would you build an experiment to ensure that the same amount of force is being applied to each egg in order to properly compare their relative? What about the strength of small/medium eggs vs. that of extra-large eggs?