Re: non-Hookeish springs

[BRIGGSP@Citadel.edu]

> Rubber bands are another good non-Hookeish spring with characteristics
> similar to car springs. I once studied their curve for purposes of
> designing a "banana car" for a local science museum competition. The
> purpose was to build a rubber band powered device (we call them
> "elastic bands" here in Canada) which, using two standard rubber bands,
> could carry a banana the greatest distance over a flat track within a
> prescribed width. It was worthwhile to take account of the non-Hookeish
> characteristic.

> Leigh

	For several years we've spiced up our intro lab on "springs and
elasticity" with an experiment in which we load down a rubber band with
increasing amounts of mass until the band breaks.  Two important results from
our observations:

	1 )	The rubber bands seem to have two "spring constants": i.e. the
graph of stretch vs. added mass looks like a straight line up to a point, then
makes a transition to another straight line (with a smaller slope).  My
interpretation is that the polymer chains stretch until they reach a maximum
distortion; after that, additional stretch is due to the typical expansion
effects that you get from most any solid material (or, as we put it, "it
changes from a spring into a string.")

	2 )	I have performed this experiment with groups all the way from
third graders to school teachers to engineering and physics majors.  I first
ask them how much weight they think the rubber band will hold before it breaks. 
Across the spectrum, almost everyone guesses TOO LOW by about a factor of
three.

	Has anyone else tried this experiment?  Have you seen evidence of two
spring constants?

Regards,	PRB

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