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Weak and Strong Materials
Name: Lorraine for Rachel
Status: Student
Grade: K-3
Location: N/A
Country: USA
Date: February 2006
Question:
Rachel just turned four years old and is in
kindergarten. She wants to know why is it that some materials are
weak like glass, and shatter and others are strong like wood or metal
and do not break easily. She is wondering what makes them so
different. I am her grandmother and I do not know how to answer this
question, but I think it has to do with the molecules/atoms being
closer together or something. I do not know how to explain it to a
four year old.
Replies:
There really is not an easy straight forward answer to this type of question,
whether it is a 4 year old or a 40 year old. Just the definition of weak
and strong are subject to debate. Glass is strong when it comes to tensile
loading (like pulling on the ends), but is weak when it comes to impact
loads (shatters when it hits the floor). Wood is real strong as long as the
load is at a right angle to the wood fiber, but if you load it going the
direction of the wood fiber, it can split on you. That is how you split
logs for your fireplace: along the fiber length. Metals are pretty strong
both with impact and pulling on it or compressing it, but has issues of its
own. Concrete is only good for compression loads. So you see, how one even
defines strong and weak can give you different answers to different
materials. Even with all of this preface, I will try to answer you
granddaughter's question the same way I would my 4 year old.
Different materials have different characteristics that cause them to be
used different ways. The way the atoms are put together and the recipe for
making the materials help define how they will behave. Glasses (or
ceramics) are great for strength, but don't drop them because they are
brittle. That means the atoms are held together real tight, but even a
small crack and a quick load can make them shatter. Metals are great to
handle these kinds of problems, but they can be heavy because of all the
things needed for the recipe, not to mention they can rust or corrode away.
Wood can handle great loads as long as they hold the load the right way.
Wood is made up of fibers (like a rope) that all run in one direction. Run
a whole bunch of fibers all in the same direction (like running a whole
bunch of ropes in the same direction) and the wood becomes strong to hold
things. Try to sit something on top of the wood fibers (not along the
length) and the wood could collapse. Plastics are great because they are
kind of like a glass and metal combination, but do not get them too near
heat. Plastics are made up of a whole bunch of plastic chains called
polymers. But get them too close to heat and the chains will unravel. As
you can see there really is not a great strong or terribly week material.
You just have to use the right material for the right job.
Hope I helped,
Christopher Murphy, P.E.
Mechanical Engineer
Air Force Research Laboratory
Hi, Lorraine.
You are definitely on the right track. It all depends
how the materials are put together. Different materials are put
together different ways.
In general there are metals, organic materials, and inorganic
materials. I will talk a little about each.
Metals are similar in that they generally transfer heat well, conduct
electricity, and are somewhat ductile (i.e. they "give" or "yield" a
bit before breaking).
Metals are composed of little crystals. When metals "give," the
crystals slide past each other a bit. They do this until they cannot
"give" anymore, then they break apart. If you mix other metals or
materials in with a pure metal, you have made an "alloy." The extra
stuff you added is like throwing rocks in between a pair of rollers -
it tries to stop the crystals from sliding just like the rock tries to
stop the rollers. This typically makes the metal stronger and more
brittle.
Wood is an example of an organic material. Most organic materials are
made of cellulose fibers made from what looks like little blocks. The
blocks are the walls from the cells of the living plant or tree. When
the cells die, the cell walls are still there. These cell walls form
long fibers. The fibers grow together to form bigger fibers in such a
way that they resemble ropes made out of many smaller strings. These
fibers grow in one direction in general, but there are also fibers
linking them together, so the material has different strengths in
different directions. Wood has "give" to it because the fibers move
out of the way when, for example, you put in a nail.
Organic materials rely in some way on bonds between carbon atoms, which
are very strong. The cellulose that makes up wood is held together
that way. When wood breaks, it is typically the little cellulose
blocks tearing away from each other, or the fibers separating, and not
the carbon bonds breaking.
Inorganic materials are usually held together by strong molecular
bonds, but do not have a crystalline structure. Thus they cannot "give"
nearly as much as metals, so they are brittle. They are very useful,
though, because they are very hard, resist wear, and are good
electrical insulators and withstand high temperatures. Glass and
porcelain are examples.
Next, a little terminology. The items you mention as being "weak" like
glass, are actually relatively strong if you pull on them the right
way. They just do not have much "give" and so break easily. We call
these materials "brittle." Other materials like pure aluminum and
annealed stainless steel stretch and twist a lot before breaking. We
call them "ductile." Sometimes, you will hear someone talking about a
material's "toughness." It is the same thing. Brittle materials have
less toughness, while ductile materials have more. The amount of
toughness a material has can change with temperature (some metals
exhibit a sudden change at very cold temperatures).
Hope that helps!
David Brandt
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