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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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