[Krampf Experiment, Stretching the Facts

Experiments experiments at krampf.com
Fri Jan 28 15:11:59 EST 2011

Robert Krampf's Experiment of the Week
Stretching the Facts

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This Week's Experiment: Stretching the Facts

I love finding things that are so common that we never really think about them, and then finding all the cool science behind them.  This week's experiment is one of those.  To explore this, you will need:

- a rubber band or a balloon
- a piece of string about a foot long 

Lets start with a rubber band.  Hold one end in each hand and pull.  What do you notice?  The rubber band stretches easily, getting longer and thinner.  You can probably stretch it to two or three times its original length.  Now try the same thing with the piece of string.  Does it stretch like the rubber?  No.  Once it pulls tight that is it.  No stretching.

What is different about the rubber that makes it stretch?  To answer that, we need to play with the string.  This is easier if you have two people, but one person can do it.  You want to hold one end of the string, and then start twisting the other end.  Keep twisting and twisting.  Fairly soon, you should notice that the string starts to kink and twist around itself.  Keep twisting.  

Once the string has quite a few kinks in it, hold the ends and pull them apart.  The string stretches!  Move your hands closer together, and the string will kink up again, making it shorter and thicker.  As long as the string remains twisted, it will kink and unkink, making it stretchy.

That is exactly what is happening inside the rubber.  Rubber is made up of very long, string-like molecules called polymers.  Those polymers are twisted and kinked, just as you did with the string.  As you pull on the rubber band, you straighten out the kinks in the polymers, just as you did with the twisted piece of string.

OK, one more thing to try.  Think about blowing up a balloon.  What is the first thing that you do, before you start blowing?  You stretch the balloon a couple of times.  Why?  Why does stretching it make it easier to blow up?

Go back to your string.  If you did not pay close attention, you may have to start over and retwist your string.  When it is all kinked up, start pulling on the ends, and notice how hard you have to pull to straighten the string.  Let the string relax and re-kink.  Then stretch it again, noticing any differences.  

You should find that the first time you have to pull a little harder to straighten out the string.  That is because the kinks are twisted over each other.  After stretching and then relaxing, the kinks are more orderly, making it easier to unkink them the next time.  The same is true for the rubber in a balloon.  The first few stretches make it easier to unkink the polymers. 

Have a wonder-filled week.

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Copyright © 2011. Robert Krampf's Science Education
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