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Title: Compression and Tension
Principle(s) Investigated:
- understand that tension and compression are distributed by various shapes in architecture
- discover that triangles provide strengthening ability for structures
- discover that arches utilize compression to strengthen bridges and other structures
- discover that arches and domes provide weight-bearing ability for structures
- discuss examples they have seen of triangles, arches, and domes in architecture
Student Expectations:
1) Students will compare the strength of domed structures to other structures.
2)They will test the strength of eggshells and discuss why they are so strong, explaining it is due to the dome shape.
3) Students will design and test their own domes using brittle materials.
- The motion of an object is determined by the sum of the forces acting on it; if the total force on the object is not zero, its motion will change. The greater the mass of the object, the greater the force needed to achieve the same change in motion. For any given object, a larger force causes a larger change in motion. (MS-PS2-2)
- All positions of objects and the directions of forces and motions must be described in an arbitrarily chosen reference frame and arbitrarily chosen units of size. In order to share information with other people, these choices must also be shared. (MS-PS2-2)
- For any pair of interacting objects, the force exerted by the first object on the second object is equal in strength to the force that the second object exerts on the first, but in the opposite direction (Newton’s third law). (MS-PS2-1)
ETS1.B: Developing Possible Solutions
ETS1.B: Developing Possible Solutions
- A solution needs to be tested, and then modified on the basis of the test results, in order to improve it. (MS-ETS1-4)
- There are systematic processes for evaluating solutions with respect to how well they meet the criteria and constraints of a problem. (MS-ETS1-2), (MS-ETS1-3)
- Sometimes parts of different solutions can be combined to create a solution that is better than any of its predecessors. (MS-ETS1-3)
- Models of all kinds are important for testing solutions. (MS-ETS1-4)
ETS1.C: Optimizing the Design Solution
ETS1.C: Optimizing the Design Solution
- Although one design may not perform the best across all tests, identifying the characteristics of the design that performed the best in each test can provide useful information for the redesign process—that is, some of those characteristics may be incorporated into the new design. (MS-ETS1-3)
- The iterative process of testing the most promising solutions and modifying what is proposed on the basis of the test results leads to greater refinement and ultimately to an optimal solution. (MS-ETS1-4)
Materials:
Eggs, Egg Carton, Plastic Plate, Scissors, Bucket, Water Beaker (at least 100 mL), Scale, Large Tray (for water and egg guts)
Procedure:
1) Measure the mass of the plastic plate, the amount of water in 100 mL of water, and the bucket. Record this information.
2) Cut the cups out of the egg carton to act as holders for the eggs. (Cardboard egg cartons are best.)
3) Arrange the eggs in a triangle shape inside the tray. Place the plastic plate onto of the eggs and then the bucket on top of the plate.
4) Add water 100 mL at a time to the bucket and record how many you add. Keep adding water until at least one egg completely breaks (the plate touches the surface).
5) Using the mass you measured for the plate, bucket, and water (total number of mL of water added*mass of 1 mL of water) calculate the total force pushing down on the three eggs when at least one of them broke.
6) Repeat steps 3-6, arranging the eggs differently. Try a smaller or larger triangle or laying your eggs on their sides. Record this information.
Student prior knowledge: compression and tension
Explanation: Give a thorough explanation of the experiment or demonstration. Your explanation should be written to give your fellow teachers a solid understanding and include greater detail than what you might provide for your secondary students. Make certain to include equations whenever pertinent.
Questions & Answers:
1) How would you improve the design of the geodesic dome to make it stronger or sturdier? Why?
2) What are some other examples of domes in nature? Do you have examples of domes in your body? How many can you think of? How many can you come up with as a class?
3) What shape are most buildings in cities? Are they more like cubes or domes? Why do you think that is?
Applications to Everyday Life:
Arches have been used in structural engineering since ancient times. Figure 1, below, shows a Roman aqueduct (in 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 load-bearing piers that support the arch. This in turn eliminates some tension stresses in the structure.
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 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. (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 load-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 have finished your background research, make a prediction about how much mass you think an eggshell can support. Then do your experiment and find out for yourself!
http://www.sciencebuddies.org/science-fair-projects/project_ideas/MatlSci_p021.shtml#background
Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.
Videos: "How many eggs does it take to hold a car?" - Richard Hammond Special
https://www.youtube.com/watch?v=xukNq7JO4dE
Work Cited:
http://biomechanics.fieldmuseum.org/sites/biomechanics.fieldmuseum.org/files/BioGuide-Dome_0.pdf
http://www.sciencebuddies.org/science-fair-projects/project_ideas/MatlSci_p021.shtml#summary
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