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Name: Derrick
Status: Student
Grade: 4-5
Location: IN
Country: United States
Date: February 2008


Question:
I did the science experiment of which brick will be stronger depending on the size of the sand grains. I used very fine sand, fine sand, and pebbles. My results were kind of strange. The pebbles seemed to be stronger, then the medium grain, and the least strongest was the finest grain of sand. In my experiment I mixed the sand with Elmer's glue. My hypothesis was that the finest sand would be the strongest. My hypothesis was wrong. Did I do something wrong or are these results right?



Replies:
I would not give up on your hypothesis yet. You only know the results for one specific recipe -- what is true for one mix may not be true for the majority. Also, you may find different results if you use different kinds of materials.

You are exploring the properties of a "composite" material. A "composite" material is made of two or more materials that act as one. In a composite, you typically have some kind of binder or matrix material (such as glue, mortar, or resin) and one or more reinforcing materials (such as your sand, rocks, or glass fibers). How much of each material makes it the strongest depends on the materials. In your case, it is possible that each 'brick' could be made stronger by increasing or decreasing the amount of glue or sand/pebbles used. A good experiment would be to make a series of bricks from each set of materials, varying the amount of each material. You would then be able to determine which ratio gives the strong strongest brick for each set of materials. Another possibility is that the glue you used does not bind to the sand/pebbles very well. Elmer's glue is not very strong on non-porous materials (like rocks or sand). You might have better luck testing with porous materials like paper or wood. A third possibility is that you simply did not mix the materials well enough (if they are not well-mixed, you could get anomalous results). This seems unlikely, but mixing is important.

Overall, I think there are more experiments to run. If you try a few more 'recipes', you might be able to make a better assessment if your hypothesis is true or not. I would also look for a better binder-reinforcement pairing.

Hope this helps,
Burr Zimmerman



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