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Name: Lachlan McBride W.
Status: student
Age: 13
Location: N/A
Country: N/A
Date: Tuesday, May 21, 2002


Question:
I am doing a school project on ELASTICITY and rubber bands. I have seen the answers to past questions posed in this area which have been helpful. I have conducted an experiment which involved attaching a load(450 grams) to 1 band then 2 ,3,4 etc through to 20 bands measuring the distance the bands stretched in the first 60 seconds ,measured that they returned to their original size when the load was removed and recorded my results. I then did the same experiment again but this time I wet the bands with water . The same load was attached but this time the wet bands did not stretch nearly as far as they did when they were dry. Why is this? I have read that when rubber is heated it shrinks( but then return to original size when they return to room temp) and suspect that if I attached the load to heated rubber bands they would not stretch as far as the ones at room temperature -- is this the case --- has the water increased the temperature of the rubber bands or there another reason.


Replies:
Lachlan,

I do not know whether temperature is the cause for what you see, but I do know a way to find out. Do the same experiment again with ice water. Then use hot water. See whether you see a similar effect in all three water cases.

If all three water temperatures produce similar effects, temperature is probably not the main cause. It may be a chemical reaction with the water, changing the elasticity by slightly changing the chemical structure.

If hot water has greater effect(more like room temperature water) than cold, then extra heat in the rubber band is a good conclusion. If the ice water behaves more like the room temperature water while the hot water is more like dry rubber bands, then the water is taking heat away from the rubber bands to make them stretch less. You have the ability to find out.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


More likely, the wet rubber bands are cooled somewhat as the water evaporates. It is true that STRETCHED rubber contracts when heated: stretching makes the rubber strands more aligned, and heating disrupts the alignment. However, rubber also becomes less resilient when cooled, as its strands become less mobile. It might be difficult for you to control the temperature of your rubber bands in your experiment, but if you could, determining their behavior under different temperatures might make a very interesting science project.

Richard E. Barrans Jr., Ph.D.
Assistant Director, PG Research Foundation


You have several things going on in your experiments that will make the results difficult to interpret. Let us look at them:

1. TEMPERATURE RESULTS: A polymer has three types of mechanical behavior that depends on the specific polymer and the temperature. At sufficiently low temperature the material is a GLASS (The material is not crystaline, but is brittle.) Plexiglas (poly-methyl methacrylate) and solid poly-styrene (not polystyrene foam) are examples of polymers that are glasses at room temperature, but begin to soften at about 100 C. You may have seen a rubber hose or a flower shattered by cooling it in liquid nitrogen (boiling point = 77 K) and striking it on the bench top. You may also have read about the disaster of the space shuttle "Challenger" some years ago. In the investigation of the cause(s) physicist Richard Feynman demonstrated that a critical seal most likely failed upon "lift off" because the temperature at the launch site was cold enough to make the brittle and result in its cracking.

As the temperature increases, a polymer becomes elastic (that is, the polymer deforms (stretches) when a force is applied to it, but returns to its original shape (length) when the force is removed. This is called the "rubbery" or "elastic" region for obvious reasons. At still higher temperatures the polymer deforms when a force is applied, but it does not return completely to its original shape (length) when the force is removed, and remains partially deformed. This is called the "visco-elastic" temperature range. At yet still higher temperatures the polymer flows when a force is applied. This is called the "viscous" range. Keep in mind that these are temperature RANGES, not sharp transitions, and that they depend upon the composition of the material being tested. Automobile tires and rubber bands are made of basically the same "stuff" but have very different mechanical properties, because of how they are formulated.

From your description of the behavior of the wet rubber bands that the glass/elastic temperature range may be about -20 to +15 C. The evaporation of the water cools the rubber bands to a temperature closer to the glass/elastic temperature, making them more glassy (and hence harder and more rigid). This would account for your observation that the wet rubber bands do not stretch as far as the dry ones. I suspect that if you took a lunch break and allowed the water to evaporate, and the temperature of the rubber bands to return to room temperature you would see that their length had increased when the "normal" value.

2. LOAD: Your load of 450 gm may be too large for the stretching that the rubber bands undergo. Elastic materials only obey Hooke's Law (the restoring force = - K * extension) form small changes in length (extensions). The more accurate formula is: f = K*T*(x- 1/(x^2)) where 'f' is the restoring force = 450 gm (the acceleration of gravity that converts weight to mass is absorbed into the experimental constant, 'K', 'T' is the absolute temperature in kelvins, 'x' is the extension (x=L/Lo), the ratio of the stretched length and the un-stretched length.

How are you attaching the rubber bands? If you are using a "slip knot" the length isn't going to be additive (that is, the sum of the lengths of the individual rubber bands). You should connect each one to the other with a paper clip and "subtract out" the contribution of the paper clips to the length.

3. BOUNDARY CONDITIONS ON THE EXTENSION VS. TEMPERATURE: There is a more fundamental reason for the behavior you are observing, and that is what are called "boundary conditions". Boundary conditions are just variables that are held constant, either for experimental or theoretical reasons. The unusual increase in the restoring force,f, as the temperature,T, increases in elastic materials (compared to a metal spring where the opposite is observed) MUST BE MEASURED AT A CONSTANT EXTENSION (x = L/Lo). Adding more rubber bands changes the extension and obscures this necessary requirement (boundary condition). To observe the effect you need to modify your experimental set up.

First, get rid of the 450 gm weight. You might use different weighed amounts of sand instead. (Glass beads are better because that all weigh about the same, so you can measure the force by counting the number of beads. You can find glass beads at most art supply / hobby shops.). Use about 6 rubber bands (about 2 in. in length each) connected by paper clips. Suspend the spring setup in a glass tube (or a card board tube with a slit cut into it so you can see the length of the rubber bands). The purpose of the tube is to keep drafts of air from interfering with the measurements. Stand a meter stick or yard stick next to, or in, the tube so that you can measure the length of the rubber band assembly. Suspend a thermometer inside the tube about 1/2 way beside the rubber band assembly, to measure the temperature. Stuff 1 or 2 crumpled coffee filters into the bottom of the tube. Now aim a hair dryer (on "low", you do not want to start a fire!) at the bottom of the tube. The hot air will rise up the tube because it is less dense than the air at room temperature. The crumpled coffee filters are there to deflect any direct air currents from rising up the tube. Now you can heat the rubber band assembly to some temperature, and when the temperature stabilizes (You do not have to pick a particular temperature, just let it stabilize.) add or remove glass beads until the length of the extension of the rubber bands is some constant value that you are free to choose so long as it is about the same value for each measurement. It doesn't have to be exact, just close.

Now you should see the result you expected. Do not feel bad about not having done the experiment under the proper boundary conditions. When the properties of elastic materials first began to be studied, I am sure many scientists and engineers, who had a lot more experience than you, did the same thing you did initially. Good Luck.

Vince Calder


That sounds like a really interesting experiment! I am tempted to try it myself. Was the water at room temperature or was it warm or cold? Was all the work done the same day with the same rubber bands?

If the water was warm it could certainly cause the effect you describe for a brief period of time but I would expect 60 seconds to be long enough for the rubber bands to reach thermal equilibrium with the room air (unless they were very massive). If you waited longer than 60 seconds did you get the same results as before? If the "wet" experiments were done on a different day it is possible that the room temperature was warmer the second day.

How long were the rubber bands immersed in the water? Does this time make a difference in the result? If you just dip them in and do the measurement do you get the same result as if they are kept in water overnight? If you get different results it implies that the water is penetrating the rubber bands and causing different molecular interactions, possibly by filling any spaces between the randomly coiled rubber molecules, causing their motion to be slightly restricted.

Greg Bradburn



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