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This week's experiment comes from an article that I read on concrete. While
it may not sound like an interesting subject, think what your life would be
like without it. We use it for roads, bridges, dams, buildings, drainage
pipes, and all sorts of other things. The part of the article that grabbed me
talked about concrete being both strong and weak. How could something be very
strong and very weak at the same time?
Materials:
To see, you will need:
some squares of chocolate
a hard bound book
waxed paper
Plan:
Start with a chocolate bar that is flat. You want pure chocolate, without
caramel, cookies, or other yummy things. Cut four squares, each one inch
square. Put a large square of waxed paper on the floor. Place the four
squares
of chocolate on the waxed paper and then cover it with another piece. Place
the book on top of that, so that it is supported by the four pieces of
chocolate.
Now, carefully step onto the book. Put your entire weight on it. Then
step off and examine the chocolate. It should be just as it was. You can
even
measure how much it was holding up. Divide your weight by 4. That is how
many pounds per square inch the chocolate was holding up. Pretty strong
stuff! Now, pick up one of the pieces of chocolate. Grab it with both hands and
twist to see if it will break. What happened? It probably broke very easily.
Now, how is it that you can easily break something that is strong enough to
support your entire weight? For that, we need to understand the different
kinds of strength. First, we need to talk about compressional strength. That is how much force
something can withstand when it is being compressed or squeezed. That is
what you were seeing when you stood on the chocolate. Both chocolate and
concrete have high compressional strength. They are made up of solid particles
packed together. Compression squeezes them tighter together, making them
strong. The other kind of strength that we are looking at is called tensile strength.
That is how much force something can withstand when it is being pulled
apart. When you twisted the chocolate, you were pulling apart, not squeezing
together. That forced the tiny crystals of sugar and butter fat in the
chocolate apart, causing it to break. Concrete also has a weak tensile
strength.
It does not hold up well when it is twisted or pulled apart. To overcome the weak tensile strength, we often add things, such as iron
bars, to the concrete. The concrete has a high compressional strength and the
steel has a high tensile strength. The combination makes a very good building
material. I guess you could duplicate that by melting some chocolate and
adding toothpicks, but that would mess up some good chocolate. There is one last kind of strength that we want to test. We want to see how
well the substance holds up to being cut. For that, we need something
sharp. Now we could use a knife, but you already have some nice, sharp teeth. Why not use those? Just don't try the same test with a piece of concrete. From Robert Krampf's Science Education Company
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