An Interesting Twist

Posted on November 11, 2009

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IN THIS POST: Theory and practice of twisted rope and how I spent my weekend.

Much of paleolithic technology is about simple mechanics.  To many people, the word “mechanics” refers to machinery the likes of which one does not see in a paleolithic culture.  But to a physicist, mechanics is just the way various materials respond to different kinds of forces.

One straightforward example is the need for materials that can hold a rigid shape when forces are applied to them.  Easy enough.  They’re called “rocks”.  If you have the right kind of rock, it will stay in the same shape under all kinds of forces.  Go ahead.  Slam that coconut down.  The coconut will lose.  And if you get even more specific about the mechanical properties of your rocks, you can pick out ones that let you modify the shape they will hold … thus: flintknapping.  That’s one of the things I plan on doing this coming weekend.

But the topic of this post in not this coming weekend, but rather this past weekend.  What if I want a material that responds very differently to one kind of force than it does to others?  Such as, for instance, yielding easily to torques (i.e. forces that bend the material) but strongly resisting tension (i.e. force that tries to pull the material apart).  Rope fits that description.

Of course, we’d probably start out by using natural objects that can perform the function without modification.  Vines would do quite nicely.  All we have to do is invent knots and we can use vines to bind things together or pull things from a distance.

But vines are only so strong.  If the tension goes too high, they’ll break.  What if we want to pull harder?  It turns out that in many plants that can resist tension like that, it’s just one part of the plant that does it really well.  Find a plant that makes it easy to separate those fibers from the rest of the plant and you can bundle just those strong fibers together, making something that can handle a lot more tension without breaking.

But why twist the fibers?  Because it’s pretty?  Well, yes, it is.  But the more practical reason that easily comes to mind is that it holds all the fibers together.  That’s definitely much easier to work with than a bunch of separate fibers.  But there is another, less obvious, reason.  None of those fibers is all that strong on its own.  The strength of a rope comes from the fact that its many constituent fibers bear more or less an equal share of the load.  But how do we know this “equal share” thing will actually happen?  If we begin pulling on our untwisted bundle of fibers, there’s a good chance one of them will become taught before the others do.  The fibers are going to be at least a little bit elastic (i.e. stretchy), so if our fiber-on-the-front-lines can stretch far enough without breaking, then other fibers will become taught as well and share the load … but not evenly.

In the process of twisting the fibers tightly, we create a little bit of tension within the rope.  As we leave each twist behind and move on to the next, the fibers can slide against each other, equalizing minor differences in internal tension.  When the rope is finished, friction between the fibers keeps any one fiber from slackening more than the others.  So by twisting these natural fibers together, we can create something that is actually much stronger than a mere bundle would be, in practice.  The whole is greater than the sum of its parts!

I spent this past Saturday with Alexandra Thorn and Tim McCormack.  I’m very glad I did!  Not only are they great company; they’re also fountains of knowledge about the various plants we looked at in our quest for useful fibers.

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Photo by Tim McCormack (click for license)

We started by grabbing some lunch in Davis Square and set out, eating on the go, toward a certain bike path.  This beautiful bike path was lined with gardens, wild flora, and colorful fall foliage.  Tim and Alex led me down the path identifying plants left and right – explaining what they can be used for, what plants they’re related to, etc….  I retained virtually none of it.  What I did retain is a sense of how a paleolithic person might see the world differently than I do.  I’ve trained my brain to identify and distinguish numbers, words, streets that deliberately look all alike, electronic components, and all the other things that have been directly important to me.  These are things I suspect a caveman might have some trouble with.  But those plants all just look like plants to me unless someone is explaining the relevant differences to me at the moment.

IMG_7115Tim offered an example of this as we were walking.  A modern, urban human, arriving at a new neighborhood, would pass a grocery store and without even thinking about it would make a mental note of its location.  Maybe she’s not hungry now, but sooner or later she will be.  But that same person, stranded in the woods for a long time, might not make a note of the location of a tree which, though barren now, will soon bear fruit.  She’d have to change her way of seeing the world for that to happen.

As we continued down the path, both Alex and Tim stopped and appeared instantly crestfallen.  They were staring at a bulldozer and a flattened stretch of path beyond it.  This, they explained, was not long ago overgrown with all sorts of useful plants.  Most importantly to me, dogbane – a good source of fibers for making rope.  The bike path was to be extended.

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Photo by Tim McCormack (click for license)

With a few pieces of dogbane left here and there, Tim and Alex began teaching me to make rope.  We also used some milkweed which was around.  Instead of describing the process here, I’ll refer you to an “Instructable” that Tim created: http://www.instructables.com/id/Make-rope-out-of-dead-plants—-with-no-tools/.

Photo by Tim McCormack (click for license)

Photo by Tim McCormack (click for license)

On the way back, we sampled fennel seeds and nibbled on various leaves and berries.  Retiring to Alex’s apartment, she made hot chocolate and we sat around and discussed many things.  I was even served a tasty “paleolithic stew”.  It did have potatoes and beans in it.  ;-)  It was a very good day.

…and I went home with more milkweed and a bit of dogbane so I can make a more substantial rope.  And some pine needles for making tea.

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Tim and Alex went home with some dogbane plants so that they could add it to a garden.  :-)

Thank you, once again, to Alex and Tim!  And thank you all for reading!

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