Investigation to show how Elastic Bands Behave Under Load.

Investigation to show how Elastic Bands Behave Under Load

Prediction

The aim of the investigation is to determine how elastic bands behave under load. Before I begin the investigation I will carry out a preliminary experiment into the behaviour of a spring, which I believe will not act in the same way as the elastic band.

In the preliminary investigation a spring was put under load and the results were as follows:

From previous knowledge of Hooke's Law, which explains, extension is proportional to load until elastic limit is reached. The preliminary experiment with the spring shows, that they obey Hooke's Law up until the point of permanent deformation (elastic limit).

I think that the thinner rubber band will have a bigger extension than the thicker band and the longer band will have bigger extension than the shorter band.

I think the longer rubber band will have the biggest extension because the molecules within the rubber band are larger and are less intertwined and the molecules are therefore stretched further causing it to have a greater extension. Although the band is stretched the rubber band does not obey Hooke's Law. The elastic band will regain its initial shape and it behaves elastically. The thinner band will also have a longer extension because the molecules are less intertwined and so the band isn’t as strong, when weight is applied the band will stretch further than a thicker rubber. In the thicker rubber band the molecules are more intertwined therefore the band will become stronger so the band will not stretch as far. In the shorter rubber band the molecules are much smaller and are more intertwined, this causes the band not to stretch, as far so it will have a smaller extension. This graph below shows how the rubber behaves.

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The term hysteresis is used to describe the difference between 'loading' and 'unloading' curves.

When the stress is reduced from point a to b, the strain remains greater than if there was no hysteresis. (From World of Physics by John Avison.)

On no point of the graph does the rubber obey Hooke’s Law. I think this could be true for the elastic bands I will investigate.

Safety

I will wear goggles to make sure no injury will occur to the eyes.
Tuck all stools under the desk in ...

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The term hysteresis is used to describe the difference between 'loading' and 'unloading' curves.

When the stress is reduced from point a to b, the strain remains greater than if there was no hysteresis. (From World of Physics by John Avison.)

On no point of the graph does the rubber obey Hooke’s Law. I think this could be true for the elastic bands I will investigate.

Safety

I will wear goggles to make sure no injury will occur to the eyes.
Tuck all stools under the desk in case the elastic bands snap and the weight shall fall.
The experiment will be carried out under the supervision of a teacher in case of any accidents.
Keep bags under the desk in case someone should trip and fall.

Variables: Key factors to vary are:

Change the length of the elastic bands.
Change the width of the elastic bands.
Change the thickness of the band.
Extend the load.

Fair Test

I will change the length of the elastic band and I will keep the width the same. I will change the width of the elastic band and keep the length the same. I will also see how the elastic band behaves under the load.

Plan

We will stretch four elastic bands one thick, one thin (keeping the length the same). One will also be long and one will be short with the width the same. The same elastic bands will be used to make the test fair and the results will be repeated in case of any mistakes (human error) to give accurate readings. The load will be up to 1000g for each rubber band and I will work out the extension. The weights will be hung from the rubber band. The initial length of the band will be measured. The weights will be increased in steps, measuring the length that each band reaches and will then be recorded.

Diagram

Results

Changing the length, keeping width the same

Shorter Band

Length = 8.2 cm

Width = 1 mm

We repeated our results on this band.

Longer Band

Length = 14.3 cm

Width = 1 mm

We repeated our results on this band.

Changing the width, keeping length the same.

Thicker Band

Length = 6.7 cm

Width = 1 mm

We repeated our results on this band.

Thinner Band

Length = 6.5cm

Width = 0.5 mm

We repeated our results on this band.

Analysis

From my results I have found that the longer band stretches further than the shorter band and the thinner band stretches further than the thicker band.

From the graphs I have drawn, the gradient is a lot steeper for the longer than the shorter band. This shows that the longer band has a greater extension. From the graphs also none of the bands obey Hooke’s Law as the graphs curve and also the extension is not proportional to the load. The molecules in the longer are much larger and are less intertwined with each other therefore the band will be easily stretched further because the molecules aren’t intertwined enough to stop it stretching. The molecules are polymer (long) so the band therefore stretchers further than the shorter as the molecules are a lot shorter and are more intertwined so they will not stretch as far.

Also from the graphs I have drawn, the gradient again is a lot steeper for the thinner band than the thicker band. This is because in the thicker band the molecules are more intertwined and the molecules are locked together so that is why it doesn’t stretch as far. The intertwining in the thicker band is quite strong and the molecules are much thicker so it will not stretch far. In the thinner band the molecules are longer and thinner and there is less intertwining so therefore the band will stretch much further because the molecules aren’t as strongly fixed.

None of the bands obey Hooke’s Law because from the graph the lines again curve and the extension is not proportional to load, as the extension is much larger. I have also noticed that the steeper the gradient the more easier the material is to stretch.

My prediction was correct and what I found supported my prediction. I did find that none of the bands obey Hooke’s Law because of the bands elasticity.

Hooke’s Law tells us that extension is proportional to load so if we double the load the extension will double, so if the load is trebled the extension will treble. I know that none of the bands obey Hooke’s Law so from my results I will prove that this is correct.

If the load was 300 the extension is 9.

If the load was 600 the extension is 18

If the load was 900 the extension is 27

Now if I take my results from 300, 600 and 900 none of these should be correct none should treble.

Load

The extension is 9

600 The extension is 30

900 The extension is 36.7

As you can see the extension was a lot more than what it should have been, therefore the load is not proportional to extension. This is true for all of my results so Hooke’s Law is not obeyed.

Evaluation

From the experiment I obtained good results and I found the experiment most enjoyable. The experiment was safe and the plan was followed and a safe procedure was carried out. We obtained 8 sets of results showing how the elastic bands behaved under load. Our results were accurate but a few anomalous results were found as you can see from the graphs I produced. The results were accurate to 1 millimetre shown by the meter ruler. The results were reliable and proved that my prediction was correct and this proves that the experiment was carried out correctly.

The results were reliable, but a few errors did occur i.e. anomalous results. This could be due to human error reading from the ruler the wrong length; the elastic band may have been stretched too far and was about to reach its elastic limit; the ruler may not have been measured at eye level causing an error to occur or even a shaking ruler could have caused these errors but these are all the possibilities.

These problems could be overcome by clamping the ruler by the elastic band, so that all the results would be the same. This would stop any anomalous results, when reading the length and make sure that that they are read at eyelevel and checked before you record them. Also using more detailed equipment so that the results could be more reliable.

The conclusions I have made are true for all elastic bands I measured. I would have to experiment further with different elastic bands to prove that my conclusions were true. I couldn’t say for sure what would happen to other elastic bands of different lengths and widths, as it would have to be investigated further. I also could measure an elastic band to a certain length or width to see if my conclusions are true.

If I were to do the experiment again I would look at other materials to stretch such as copper, steel, rubber etc to see if any other materials obey Hooke’s Law. I could again look at different lengths and widths of other elastic bands.

I would also use accurate measuring equipment so that no errors occur and good, clear accurate results are obtained. The good results would present a clear idea of how elastic bands behave under load. Further experiments could be carried out, using different equipment to see if my conclusions are true that elastic bands do not obey Hooke’s Law.