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Re: non-Hookeish springs




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


Sure--my version of this lab is called Everything You Never Wanted to Know
About Rubber-Bands. In the first part of the lab I have them stretch the
band (but not beyond its elastic limit) and then slowly remove the weights
and measure the length. You get nice hysteresis curves. I also have them
take a 5 minute break with lots of mass hanging on the band and come back
and see that the band has continued to stretch--creep in the language of
such phenomenon. The turn-over of slopes IS where you've exceeded the
elastic limit. I have the students measure the 'vitals' of the band before
the experiment (they get a 'virgin' band right from the box) and then do a
'post-mortem' after the band has broken. Here they can find permanent
deformation.

Cautions: Depending on the type of rubber band, it can take several
kilograms to break. This puts toes (and more importantly, expensive
mass-hangers) at risk. We bought a set of mass hangers that are VERY
rugged (but not so uniform) for just this kind of experiment. Foam mats
under the masses are another good idea. It does become interesting
watching how they stack the masses once the height of the stack exceeds the
length of the mass hanger.

Rick Tarara