Science Buddies: "Ask an Expert"

With so many types of rubber bands out on the market, I'm curious about the reason for that. I also want to know what makes one better than another. So, for my science project, I'm comparing the strength of rubber bands: 1) with the same length but a different width and 2) bands with the same width but a different length to see what makes the most difference. I've purchased rubber bands made by the same company, to control my variables as much as possible, and my test is being done indoors (at room temperature). To measure their strength, I'm suspending a rubber band from a cup hook screwed into the top of a doorway. A metal ring that can be opened/closed is being attached to the suspended band, and I'm hanging a bucket beneath, so that I can add weight. To test for strength, I'm adding coins to the bucket, to see how much each type of band can bear before breaking, and I'm recording the weight of each bucket load (bucket and attached metal ring included) on a digital kitchen scale. I'm repeating the test three times for each type of rubber band, to get an average. I've seen other projects, where students were testing rubber bands; they were testing the impact of temperature on theirs, and they recorded the weight each band could bear, but they also made a point to measure the length of their rubber bands at the breaking point. My first question is this: Is the measurement of each band's stretched length necessary in my experiment? Second, I've noticed that others have used Hooke's law---which is very complicated!---with their experiments, but rubber is supposed to be a "non-hookean" substance. Am I supposed to incorporate that law (and formula) into mine? Finally, is testing the strength of rubber bands the same thing as testing elasticity? I'm assuming it is not.

Alex

Very nice experiment!! You've explained it well and have asked really good questions.

First, the length at which the rubber band breaks is not very important. But measuring the length of the band each time you add more weight would be interesting to study. Make a graph of the weight you have hanging on the band by plotting it on the x axis and plotting the length on the y axis. Study the shape of this plot. Hooke's law states that the force required to stretch a band will equal the length it stretches times a constant. If Hooke's law applies, then the plot of force versus length of the band will be a straight line. Is it?? If it is curved than your rubber bands are "non-Hookean". A simple explanation of Hook'es law can be found here: http://en.wikipedia.org/wiki/Hooke%27s_law

One important variable you should record that you didn't mention is the thickness of each band. The thickness of the band, times its width, will give you the cross sectional area. If you make a graph of cross sectional area versus weight at which a band breaks you will make an interesting discovery.

Good luck and have fun!

Mr. Neu,

Thanks for your reply! You're really smart! I appreciate your ideas, but I'm adding nickels for weight, so the increase is very slight, and I don't think I can measure and plot that. • The formula is confusing to me; you said it's force = length x "constant," but I don't know what you mean by the term "constant," and I don't think I can measure the stretch made by the addition of one nickel. • I know the thickness of each band. The ones I'm comparing are the same, but I don't understand what you mean by "cross sectional area," and I'm not sure how to graph something like that.