Apparatus:
• stand • elastic band
• Weights • Ruler
• Tray or box • Nail
Method:
To begin, you must measure your elastic band when it is not stretched. Make a note of its length.
This experiment must be carried out on a table’s edge. Place your stand on the edge of the table. Place your tray or box directly below the stand. From the stand use a clamp to allow a nail to stick out from the stand. Hang an elastic band from this nail. The weights should be on a separate stand which it self weighs either 10g or 100g, depending on which one you use. This will hook onto the elastic band and the weights will sit on this stand when needed. In order to make it easier to measure the length of the band throughout the experiment, it is helpful to selotape a meter ruler to the top of the stand so that ‘0’ on the ruler is inline with the tip of the elastic band.
Begin the experiment by placing one weight at a time on to the hook hanging from the elastic band. For each weight added, note down the size of the weights you are using, and then measure the length of the elastic band with the ruler, and note down the length. Use the table provided below.
It is best that when adding weights, they should be added so that it is a constant increase, I.e. add 10g each time. Start with 10g, and gradually increase the tension by adding 10g weights.
Keep increasing the weight/ tension on the elastic band until the band snaps, or you feel you have collected enough readings.
(It would be suggested to collect as many readings as possible with the time available. Between 500-1000 should be sufficient)
This table should be filled in throughout the experiment. When each weight is on the elastic band, note down in the corresponding column the length of the elastic band.
Safety:
When placing the weights onto the elastic band, the main stand may become unstable due to the weights, so in order to avoid hurting one’s self use a clamp to fasten the stand to the table.
The only other safety issue that needs to be addressed is if the elastic band snaps, the weights will fall. To avoid any injury or damage one should place something underneath the weights to catch them when they fall, a tray, or box would be sufficient.
Hypothesis:
I think that the elastic band will increase in length, but not at a constant rate. I think this because when experimenting with my elastic band there came a point where the band would stretch no further, even with additional force, until it broke after a strong pull. And when pulling the band, I felt that the it stretched a lot without a great deal of force having been applied. So I feel that the extension of the elastic band will not be proportional to the load, proving that Hooke’s law does not apply for elastic bands.
Also it appears to require no real effort to stretch an elastic band to a certain point, but to get beyond this point, it required a lot more effort. So I will expect there to be a large extension when only a small weight is placed on the elastic band.
Results:
Table 1 - Length of elastic band at start: 8.0cm
I have only plotted 500 pieces of data on my graph out of the 700 that I had collected. I did this because I felt that in order to fit all of my data on my graph, I would have to try and fit all of the data on and this would make my graph more complicated to plot, and to read. So to make it easier for myself I only plotted 500 pieces of data. This will not effect my results.
Analysis:
As we can see from my results in Table 1, the elastic band suddenly stretches 1.1cm when only 10g is placed on it. Once the weights begin to increase by 10g, the extension increases gradually by a small amount, but it is never a constant amount. The extension may sometimes be only 0.1cm, whereas other times it is 0.4cm. So there is no constant increase.
From the graph we can again see the gradual increase in the extension, but it shows more clearly how it is never a constant increase. At the origin, where there is no weight on the elastic band there is no extension. But when the lowest value weight is placed on it, the extension increases. But when the amount of weights goes up, the extension only increases by a small amount. This is clearly shown by the sudden change in shape of the curve at the very bottom of it.
Conclusion:
From my results I can see that the weight (load) is not proportional to the extension. I can say this as from my graph we can clearly see that it is curved, and that from the origin, there is a sudden change in shape of the graph, which is different to that of the rest of the graph. If a graph is curved it means that the x-axis and the y-axis are not proportional to each other. I.e. x does not equal y. Therefore I have proved that an elastic band does not obey Hooke’s Law, as stated in my Hypothesis.
Also in my hypothesis I mentioned that little force is required to stretch the elastic band up to a certain point. This theory is proved by my graph, as at the origin there is an irregularity about the curve. This change in shape is where the elastic band needs little effort for it to extend. So my other prediction in my Hypothesis is correct.
Evaluation:
The main problem that I came across was measuring the length of the elastic band. It is difficult to accurately measure the length as the elastic band moves as it is hanging from the stand, and so trying to find the very top and bottom of it is difficult. To overcome this it might be helpful to have a set square attached to the ruler which sticks out so that it was flat on the bottom of the elastic band, and showed the measurement on the ruler.
I would have liked to have done the experiment with different sized elastic bands, in order to find out whether the thickness effects the relationship between the load and the extension. More time was needed, as I would have liked to have collected more data than what I had in order to see whether the extension increased just before breaking.
