Ask A Scientist General Science Archive

  
name         Eric H.
age          18
    
	Question -   1st question
There are 3 questions I wanted to ask about the behaviour of rubber bands
based on an experiment I had done
in my experiment, i added weighs one by one onto a rubber band and
measured the length of the rubber band each time after waiting for 15secs
(recording A)
Then I remove the weighs one by one, and measured the length each time
again (recording B)
In my trial where the max mass of weighs is 800g (I don't think there is
deformation because the rubber band returned to its original length after
some time), I noticed something very interesting

The length of the rubber band is longer for each corresponding weigh in
recording B than recording A, ie, the length of the rubber band is
shorter when, say a 300g weigh, is added to the rubber band from its
relaxed state than it is when it had been stretched to 800g and 500g is
removed from it(300g remained on the rubber band)
The explanation I can come up with is that some of the potential energy
stored in the rubber band when it was stretched was not released when the
weighs were removed, the question what is the theory behind that?) 
-----------------
Exactly right, Eric.
     Your direction-dependent rubber-band behavior is called Hysteresis.
That term is applicable to many things where the forward trace differs from the reverse trace, not just rubber-bands.

Suppose your rubber band was a "bungee cord", a bundle of rubber-bands wrapped in a braided fabric sleeve.
Whenever it moves, whether stretching or contracting, the rubber is rubbing against the fabric from the inside,
or perhaps the crossed fibers of the braided sleeve are rubbing on each other, with some normal force,
and some force and energy is lost to sliding friction.

This explanation would have no dependence on the speed of the motion.
Other explanations do depend on the speed.
They involve things going on inside the rubber, things like entropy and potential energy, temperature,
slipping, sticking, re-orientation and vibration rates and amplitudes.

Suppose my example bungee cord was subjected to some vibration while you slowly stretched and contracted it.
The vibration would force the fabric's friction points to slip with less force applied, and less energy would be lost.
This effect would depend on the amplitude of the vibration: too small-> no effect, large enough-> 100% effect.
In nature, heat provides background vibration between molecules.
This behavior would exhibit a threshold temperature.  Cold -> much hysteresis and energy loss, Hot-> less hysteresis and energy loss.
It would also be speed dependent, whenever the vibration level was somewhere in the transition range.

Behaviors the other way around (cold->perfect spring,  hot-> lousy spring) also happen, and are perhaps a little more common.
It happens to everything as you approach melting.
You can imagine that if some of the parts of molecules, which stick together giving the rubber it's zero-stress shape,
were to gradually start slipping,
then after stress was applied some slow deformation would occur, bleeding away some of the stress.
More wasted energy.  But this time it is efficient when colder or faster, lousier when hotter or slower.
I think this is the kind of thing you are controlling by waiting exactly 15 seconds, for example.
Perhaps if you try 3 seconds, 20 seconds, and 2 minutes you would get slightly different answers.
Likely much different for 2 hours on a hot day.

There are also explanations based on entropy and potential energy interchanging
in lots of simple little heat-engine-like molecular behaviors.
Molecules like to thrash about in our room-temperature heat bath.
 if you pull harder on the end of a vibrating elastic string, it will vibrate faster and harder (more heat).
It will also stretch a little and pull harder on its ends too (stored energy).
Confining away some percentage of the space required for vibrations to occur will take some addition of energy.
You are squeezing the box containing a bouncing ball, and it makes heat.
 And if you relax the stress gradually, heat will give some of that energy back.
If you relax too fast, maybe it will not.   Or too slow.   How fast is too fast?  Sorry, I do not know the details.
Something to do with the thermal capacity and/or thermal conductivity of the rubber, for one thing.

Perhaps you could do some library research to find out more specifically about elastic losses in rubber,
or expand your experiment to include a range of temperatures and cycling speeds.

Cordially,
Jim Swenson
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Unless the rubber bands were very large, you probably exceeded the elastic
limit of the rubber so that the rubber band was permanently deformed. In
order for the process to be reversible it is necessary to use small weights
that do not stretch the rubber band very much. You can test this out by
adding small increments of weight -- say 30-40 gm and measure if the
extension is proportional to the weight added. If there is any deviation
from linearity, you probably have exceeded the elastic limit of the rubber
band.

Vince Calder
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