Tuesday, August 29, 2006

Why do rubber bands stretch?

I continue to be amazed at how interested Chase is in what I'm doing. I can actually see her process everything I do, take note, and then file it away.

I've always believed you should address your child with respect, and be especially careful not talk down to them. Tell them what you are doing. Explain things to them.

So we do our best to explain why things float in the bath tub, what shadows are, why it is warmer in the sun, why is the sky blue, etc. I've personally found it to be a good excercize to explain complex concepts as simply as possible.

We were doing pretty well until I came up with the question: why do rubber bands stretch?

The simplest answer I came across is as follows:

"you can think of the polymer molecules (that comprise a rubber band) as a tangled mass of very long spaghetti strands tacked together to one another in a few widely separated places along the strands. When the rubber band is stretched, this tangled mass of "spaghetti strands" stretch out and line up parallel to one another (more or less / mostly less). It is this lining up that allows the rubber band to stretch. However, there is a limit to how much the rubber molecules can be stretched because they are attached to one another in these few places. (The technical name for these links between the chains is "cross linked".) If too much force is applied these cross links will break and the rubber band will "snap". - Vince Caldor, Ask A Scientist (http://www.newton.dep.anl.gov/askasci/eng99/eng99163.htm)

For those of you who are a little more math and science inclined:

"At a constant temperature, and for elongations not too large, a rubber band obeys Hooke's Law: The force, f = - K(X - Xo) where K is a constant, (X - Xo) is the elongation and the sign in negative because the force is in the direction opposite to the extension. That is the force, f, is trying to pull the rubber band back to its equilibrium length, Xo." - Also from Vince.

Vince goes on to say:

"When you heat almost any solid, it expands. But not rubber bands!!! The reason is that heating the rubber molecules make them move around more. They become less aligned as a result and the rubber band SHRINKS instead of expanding when it is heated!!!" (This is quite exciting, as seen from his use of exclamation marks)

But enough about Vince (I found him slightly condescending, but I'm sure it's just because he's one of those super "mathy" types who doesn't understand why everyone doesn't know calculus) and why rubber bands stretch (although I'm sure you're all happy you now have something stimulating to talk about at your next party). I came across a fun Web site while I was doing my research and wanted to share it with you.

Check out: http://www.why-is-the-sky-blue.tv/why-is-the-sky-blue.htm

*Rubber band image courtesy of Wikipedia. The entry on rubber bands is fairly hilarious.