Investigate Whether Elastic Bands and Springs Behave the Same Way.
SCENARIO:
Investigate whether elastic bands and springs behave the same way.
AIM:
When a load(weight) is applied to either an elastic band or a spring, do they return to their original length i.e. is the extension directly proportional to the load.
BACK GROUND INFORMATION:
The structure of elastic bands and springs are different. By studying a Newton metre, I noticed that the marks between each Newton applied were equidistant. This implies that a spring returns to the original length, therefore the load and length are directly proportional. This is because the structure of a spring is coiled. Therefore the molecules have been twisted and pulled apart, this creates the spring, as the molecules try to pull themselves back together. An elastic band is made of polymers.
Hooke's Law states when a material is stretched, the change in length is directly proportional to the stretching force applied. However when an elastic limit is reached the material will be deformed and no longer obey the law. When a material is stretched, the molecules are pulled further away from each other, weakening the bonds, if the bonds break then the molecules cannot return to their original state. This is known as the elastic limit. Therefore, the number of molecules will effect the elastic limit of a material. The more molecules there are the more energy is needed to weaken the bonds. When a load is applied this creates gravitational potential energy(g.p.e). Therefore, materials smaller in width and length need less weight for the elastic limit to be reached.
VARIABLES
There are several variables which when altered will effect the behaviour of elastic bands and springs. To make a fair procedure their are certain variables that need to be controlled, and others that need to be varied in order conduct the experiment and give a detailed conclusion.
The room temperature will effect the experiment, if it drops the particles will vibrate less, and will be drawn closer to each other, meaning the bonds will be harder to break and the material will contract. However if the temperature is raised then the heat energy will be transferred to the molecules, as the molecules vibrate more the bonds to weaken, resulting in the material expanding and become more stretchable. The bonds will also weaken if the band has been used frequently (this will heat the band and spring as well), to prevent this I will use 3 different elastic bands and springs of the same size and width. Due to limited resources I cannot control the temperature, therefore preventing me from experimenting with this variable.
A possible variable I could alter is the width, size or cross sectional area. This will require different amounts of energy to stretch the band or spring. If the width and size increase then more weight is needed to create more gravitational potential energy in order to separate the bonds. However I have decided not to use this variable, as the range of results would be limited because it would be difficult to alter these variables to a large degree.
For my experiment I have chosen to investigate whether elastic bands and springs return to the original length, after a load has been applied. To discover if the elastic band and spring return to the original length, results will be recorded when the weights are being unloaded. I have decided to experiment with this variable, as I have the resources and there will be a large scope of results for this investigation.
A pilot study will be administrated in order to find the number of intervals and how much weight should be applied per interval.
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For my experiment I have chosen to investigate whether elastic bands and springs return to the original length, after a load has been applied. To discover if the elastic band and spring return to the original length, results will be recorded when the weights are being unloaded. I have decided to experiment with this variable, as I have the resources and there will be a large scope of results for this investigation.
A pilot study will be administrated in order to find the number of intervals and how much weight should be applied per interval.
PILOT STUDY
I have conducted a pilot study in order to familiarise myself with the experiment, to make modifications if needed, to discover the elastic limit of an elastic band and spring, and to determine values needed for the experiment.. Upon viewing a Newton metre I discovered that the maximum allowance of weight was 10 Newton's, and that there were markings for every 1 Newton applied. I decided to apply 1 Newton at every interval, at 11 Newton's the elastic limit of the spring had been reached, it became uncoiled and unable to return to it's original state.
A thin, short elastic band was chosen, I applied 1 Newton at every interval the elastic limit of the band was 19 Newton's, at which point it snapped. A longer, wider, band with a greater cross sectional area was then used in the same manner. The elastic limit was at 27 Newtons. I decided to use a thin short elastic band, as if a longer, wider band was used more weight would be needed to be applied. Therefore the band would not stretch as much in the first 10 Newtons as opposed to the thin short elastic band.
I decided that the same amount of weight should be applied to both elastic band and spring. Therefore 10 Newtons will be applied to both. 1 Newton will be applied at every interval, as this will give a wide range of results and will not be to time consuming.
A different, however similar sized band and spring shall be used. This is because if the same band or spring is used then the bonds will progressively weaken. To make the test fair, and the results accurate, three measurements will be taken for each result so that an average can be taken. The other variable to control is the room temperature, however as already stated due to lack of resources this will be difficult. Attempting to minimise the difference in temperature, the experiment will be conducted on the same day, and away from the windows and doors to prevent draft. In order for precise results to be gathered, they will be recorded to the nearest millimetre. To further increase the accuracy, the apparatus will be utilised properly. The metre ruler will be clamped parallel to the spring or elastic band. A pointer will also be used, and readings will be taken from eye level, once the spring or elastic band has stopped bouncing. While conducting the test I noticed, that after every bounce the band stretched more, or began to creep To prevent this when the experiment is conducted I will prevent the band from bouncing and then swiftly record the result.
The safety of the experiment will also be taken into consideration. The apparatus will be checked, to insure there are zero errors. Safety goggles will be worn in case the elastic band or spring snap. The stand will be clamped to the desk so that it does not fall over or move forward. I will also behave in an appropriate manner, making sure not to nudge the apparatus.
After completion of the pilot study, I proceeded with the experiment and assembled the following equipment;
APPARATUS:
The following equipment will be used;
* Spring(3) [ 2cm width, 5cm length]
* Elastic band(3)
* Stand
* Pointer
* Hook
* 1 newton weights
* Clamps
* Metre ruler
* Safety goggles
METHOD:
) Take necessary safety precautions.
2) Assemble the apparatus (using a spring), check the equipment for faults, and then I will assemble(as illustrated in the diagram)
3) Clamp stand to desk so to stabilise the equipment, and insure it cannot be knocked over
4) The weights will then be applied steadily, 1 Newton at time.
5) Stable the weights, once at rest read metre ruler parallel with eye level and record the result.
6) Repeat these steps, increasing the weight by 1 Newton at a time until a total load of 10 Newtons has been applied.
7) Unload the weights 1 Newton at a time and record the results.
8) Alternate between using an elastic band and spring until three sets of results have been collected for each.
9) Repeat any anomalous results, before dissembling the equipment.
DIAGRAM
PREDICTION:
I know a spring obeys Hooke's law through observing a Newton Metre, where the marks are equidistant from one another. Therefore a spring's extension will be proportional to the load. Therefore I conjecture a straight line graph through the origin will be produced, similar to the one illustrated, once the elastic limit has been breached a curve will occur as less weight is needed to stretch the spring as opposed to before, due to less energy needed, as there are fewer bonds.
However the structure of an elastic band and spring are different, a spring is coiled, I therefore conjecture that the two will behave differently resulting in the elastic band disobeying Hooke's Law, by not returning to the same length. Therefore a graph similar should be produced;
OBTAINING EVIDENCE
Through conducting the experiment the following results were gathered;
Elastic Bands
Length (Centimetres)
Average Length (CM)
Load (Newtons)
Loading
Unloading
Loading
Unloading
0
2
2
2
5.5
4.7
5.2
2
5.1
7
7.5
8
0.3
0
1.3
7.5
0.5
2
0.5
1
1.3
2.5
2.3
3.3
1
2.7
3
3
3
4.5
6.3
7
6.8
3.5
6.7
4
5.5
6
6.5
8
8.5
9.3
6
8.6
5
6.5
7.3
7.8
20.3
9
20.5
7.2
9.9
6
9
20
21.7
23.8
22.5
23.8
20.3
23.4
7
23
23.3
24.2
24
24.3
25.3
23.5
24.6
8
25.3
26.3
26.8
26
27
27.5
26.1
26.9
9
26.8
27.5
28.3
27.3
28
29
27.5
28.1
0
28
28.5
29.5
28
28.5
29.5
28.7
28.7
Springs
Average Length (CM)
Load (Newtons)
Loading
Unloading
Loading
Unloading
0
2
2
2
2
2
2
2
2.0
5.3
5.3
5
5.5
5.4
5
5.2
5.3
2
8.8
8.8
8.3
9
9
8.5
8.6
8.8
3
2.3
2.3
2
2.5
2.5
1.7
2.2
2.2
4
5.5
5.5
5.3
5.5
5.8
5.3
5.4
5.5
5
8.8
9.2
8.6
9.3
9
8.8
8.9
9.0
6
22
22.6
21.3
22
22.8
21.5
22.0
22.1
7
25.3
26
24.5
25.3
25.8
24.8
25.3
25.3
ANALYSIS
On the previous pages I have collected my results, and displayed them as graphs. Firstly I shall comment on the graph for a spring.
A straight line graph of a positive correlation was produced, illustrating that as the load increases the extension is directly proportional, as stated in Hooke's law, hence the spring returns to the original length. This is as I had predicted, due to the iron material when the molecules and the bonds are twisted, the molecules attempt to pull themselves back together. This creates it's spring. When the bonds are broken, the spring can no longer return to the original state, therefore it no longer obeys Hooke's law.
The spring however forms an "s" shaped graph, this illustrates that an elastic band does not obey Hooke's law, as by studying the gradient of the graph the extension and load are not proportional. As the bonds have weakened they no longer return to the original length, hence the "S" is formed. This again shows Hooke's law does not apply.
This is due to the elastic bands structure. As stated in the prediction this consists of polymers. The horizontal bonds linking the chains are strong, whereas the vertical bonds are weak causing the molecules to slip over one another, thus allowing the elastic band to stretch. The elastic band cannot return to the original shape as the bonds have been weakened.
The area between the curves is the hysteresis loss which represents the energy needed to separate the polymers. By finding the area of a "curved" curve the hysteresis loss is given.
EVALUATION
Overall I feel that the experiment was a successful as a range of results were produced, which a conclusion was able to be formed from. The results were as accurate as I could have personally obtained, with the majority agreeing to the line of best fit graph. However the results could not be considered reliable, with a few anomalous being produced, due to human error and limitations incorporated within the method.
Although I took necessary procedures and conducted myself in the appropriate manner, mistakes still occurred. For instance when measuring the elastic band and spring, although done at eye level, a great deal of accuracy could not be achieved. Also as I applied the weight, bouncing was caused, no matter how steady I applied the load. On every bounce the elastic band crept, i.e. the elastic bands length in creased. This was due to the elastic band's energy dissipating, due to the Gravitational Potential Energy being transferred into Kinetic energy, which was then transferred back into Gravitational at the top of the bounce, this process is repeated until all the energy has been dissipated. Therefore some elastic bands were measured with different levels of energy, meaning the bonds were either weaker or stronger, meaning the band had not stretched it's potential length.
Another major factor to be taken into consideration, as discussed in the plan, is the room temperature. An increase in temperature would create weaker bonds, and expansion in the material, whereas a decrease would cause the opposite with the retraction of material and molecules, causing stronger bonds.
To improve these limitations in my method, the experiment could have been performed in controlled temperature conditions. However this would be too expensive. In order to reduce human error, is the incorporation of advanced technology, to measure the length of the elastic band more precisely, and load the weight in order to minimise bounce. If the same elastic band and spring were used on the three occasions, then the bonds would gradually weaken. By using different elastic bands and springs however, the surface area, length, width, may have varied, which would cause various amounts of molecules If three identical but unused elastic bands and springs had been used, then the results gathered would have been more accurate.
Another limitation in my conclusion is that the results cannot be taken generally, my aim was to discover if elastic bands and springs behave in a similar manner. The experiment conducted was of only one situation. Other experiments could be conducted to discover if elastic bands and springs act similarly in different situations, such as coiling the elastic band, applying the load to elastic bands and springs conducted in parallel, or series. Or the variables could be altered for example exploring how much elastic bands and springs expand/contract when various levels of heat are applied, when the load and size are kept the same, or altering the width, size, and cross sectional area, when the load and temperature are kept constant.
BIBLIOGRAPHY
* GCSE NEAB Syllabus
* CGP Physics/Chemistry Revision Guide
* Complete Co-ordinated Physics (Heinman)
* PASS Physics- key facts (AJ Flint BSc MSc)
* CD - ROM Infopedia U.K
John Egan
1BN
