Every time it is stretched the structure becomes less tangled, thus making it longer. For this reason, I believe for each set of results the extension will increase slightly.
Apparatus
- Clamp and stand
- Rubber band
- 1g weights and holder
- G clamp
- Meter ruler
- Mirror
- Pin
Safety
I will ensure that the following things happen, to ensure the experiment is safe: -
- Safety glasses are worn while the rubber band is being stretched.
- Rubber bands are handled carefully
- Mirrors are handled carefully
Plan
Set up the apparatus as shown in the diagram below: -
I did some preliminary work to use a thin rubber band, as the thick band did not extend enough to obtain good results. (5N on the thick band = 5.35cm extension, whereas 5N on the thin band = 9.1cm extension.)
- Add 1N of weight to the rubber band and record its start position on the ruler.
- Use the mirror to gain accurate results from the needle. (To do this, make sure the needle’s reflection cannot be seen. This means that your eye is on the correct level.)
- Record the results (Newtons over extension)
- Repeat 1-3, but add 1N to the loads each time
- When the load reaches 10N unload the weights 1N at a time and record each result like before.
- Repeat the whole experiment until you have three sets of loading and unloading results.
Obtaining Evidence
There are no anomalous results, all given results where used in calculating the average extension.
Analysis
My prediction was correct in the fact that the extension did increase as the load did, however, I was incorrect in saying that they would increase in proportion. This is not so, as we can see from the graph (it is a curve). This also means that my predicted graph is wrong, as I predicted a straight-line graph. I was unable to prove my point that the band would reach its elastic limit and snap, because the maximum load I was using (10N) did not go over the limit. Here are my graphs; there is a hand written copy of each for accuracy, to compare with my predicted graph.
However, one part of my prediction was correct. As you can see from the results, each time I repeated the experiment, the rubber band’s extension grew slightly. This is because (as I explained in my prediction) the structure of rubber, a polymer, is very tangled and as it is stretched the structure becomes less tangled making it a little longer each time you stretch it. It is for this reason that rubber bands wear out and either become hard or loose their elasticity over time. In conclusion the rubber band does behave a lot like the steel spring, in that, the coil of a spring can become elongated because of the stretching force we place on it. However, because a metal’s structure is not like a polymer’s, this does not happen until you have reached the elastic limit, when the spring will be overloaded and stretch beyond is nature length, never going back to its original length. Whereas, when a rubber band reaches its elastic limit it simply snaps and cannot hold any weight.
Evaluation
I think my results were fairly accurate and good enough to have tested my prediction, as I have done by proving it to be wrong. I performed a fair test, as there was a dependent variable, an independent variable and the rest were fixed. This is shown by the three sets of results being more, or less, the same (except for, as I predicted, the band would stretch a little more as the experiment progressed). I used a mirror to ensure that I was reading the ruler at the right level. However, there may be a slight problem with this method. In order to gain accurate results from this mirror method, you must make sure that the mirror is parallel with the ruler, otherwise the angle may be wrong. This is something I think I could improve on if I was to repeat the experiment. There were no anomalous results that I could pick out, so the procedure I carried out was quite good. Another thing I would change about that procedure, however, is the pin used as a pointer onto the ruler. I stuck it to the top of the weight holder with plaster scene. I felt this was not the best way of doing so and would use cello tape instead. This is because I would have been able to ensure that the pin was at a right angle to the ruler and the cello tape would have stopped the pin from moving which was impossible while using plaster scene. Testing another type of elastic band or investigating the same thing with a steel spring could have furthered the experiment, so that you could directly compare the results. Also, preliminary work to test elastic limits the bands’ (or springs’) would have given us better graphs, as I could have got a better curve.