Hypothesis
My hypothesis is be based on the secondary data into springs and the theory into Hooke’s Law. This put together will enable me to produce a fairly accurate prediction.
What I think is that the thin wire will return to its original length, providing, that the strain exerted on it does not go beyond its elastic limit. If this does happen then the wire will become inelastic. But if the force keeps increasing then the wire will eventually snap.
While the wire is still elastic the force being exerted should be directly proportional to the extension. As stated in Hooke’s Law.
Plan
I am going to test weather Hooke’s Law applies to thin wire. This is going to be done by doing an experiment where different weights/forces are added to a piece of fishing wire. The thickness of the wire will be kept at a constant and the sole variable of the experiment will be the force that will be added. The weight will be added until the wire breaks.
The experiment will be made a fair test by ensuring that every factor is controlled. The thickness of the fishing wire is very important and should be kept constant throughout the three experiments. If this is changed then it will ruin the whole investigation and mean that all the results are false. The weight/force added each time needs to be the same as well so that the tables and graphs are easier to read.
The experiments will have to be repeated in order to obtain more accurate results. I have decided that three is a good observation range and should eliminate most errors. The results will differ from one another so I will work out an average.
The safety of myself and others is very important in the lab. In order to make the experiment as safe as possible I am going to ensure that the following requirement is satisfied:
•Everybody must wear safety goggles just in case the fishing wire breaks.
Apparatus
• Thin Wire
• Metre Rule
• Stand
• Clamp
• Goggles
• Weights
Method
This is an investigation into Hooke’s Law. In order to prove this theory right or wrong I am going to carry out a practical investigation. It will be carried out in the following order:
• Place stand on a bench and attach the metre rule using a clamp, ensuring that it is perfectly vertical.
• Cut piece of fishing wire about 60-75 cm in length.
• Attach the wire to the clamp, leaving a loop at the bottom in order to hang the force.
• Measure the length of the again.
• Start putting on a force. Two Newton's every time.
• Measure the extension after each weight is put on.
• Repeat the above three time to ensure accurate results.
Table of Results
Below are the results for the practical I carried out into thin wires:
Fig 1.5
For the graph containing this information refer to the back of the investigation,
Fig 1.6 and 1.7.
Conclusion
From my results I can establish that the fishing wire is elastic roughly up to when the extension is about 50mm and the force about 6N. It is not directly proportional throughout that Elastic stage but is roughly proportional.
The wire is directly proportional in Extension 1 and 2 until 40mm in extension and 4N in force. The line of best fit then curves away which represents that the wire has become inelastic. From this I can also say that the elastic limit for Extension 1 and 2 is 40mm.
Extension 3 seems to be an anomalous result. This is because it does not seem to fit in with the general pattern of Extension 1 or 2. The elastic limit is reached when a force of only 2N is applied. After the elastic limit, the inelastic stage of the wire is also wrong because the curve is not going constantly toward one direction, it goes side to side. The difference in the curve is not huge but it is noticeable. The graph slightly resembles that of rubber stretching when a force is applied (Fig 1.8). Although Extension 3 does not sway as much as rubber it still does resemble it somewhat.
In Fig 1.7, which is the average extension of 1,2 and 3, the results are similar to the graph labelled Fig 1.1 (which shows Hooke’s Law in practise). This would indicate to me that the results do fit in with my original prediction. This was that the force and extension will be proportional to each other until the elastic limit is reached, at which point the wire will become inelastic and no longer will the extension or force be proportional to each other. This is what has happened but the results do not fit the pattern exactly, because the extension and force are
only roughly proportional. The reason for this is that Extension 3 is wrong.
After the elastic limit when the thin wire becomes inelastic, the line of best fit is still wrong. Between 6N and 10N the line dips meaning that the result for 8N must be anomalous. If you disregard this point and continue the line of best fit from 6N to 10N then the graph makes more sense. The line missing out the point for 8N is represented by a pencil line on Fig 1.7.
Evaluation
The evidence from my results are not good enough from which to draw a firm conclusion upon.
There is one anomalous result on Fig 1.7. This is the result for 8N. The only possible explanation is that the results for Extension 1,2 and 3 were wrong as well. On Fig 1.6 all the results for Extension 3 are wrong. Again the only explanation is inaccuracy in measuring. For all the results that are given they are to the nearest 10mm because it was difficult to measure the wire. The reason for the reading difficulty was because the wire was about 3/4cm away from the rule and had to be held by hand. If the experiment was repeated than a more accurate measuring device would have to de used. Such as a ruler which has a more accurate (mm) scale. This should then eliminate a slight change in the results. In order to gain the most accurate results then whole experiment will have to be repeated, but using a different method. This will then hopefully enable us to draw firm conclusion for the Hooke’s law investigation.
The alternative method to do the experiment - and do it more effectively with a grater accuracy of results - would be to do it horizontally instead of vertically. This method is displayed by Fig 1.9.
Doing the experiment horizontally rather than vertically, as the diagram shows, would make reading the extension easier thus making it accurate. The accuracy could be made even better if a more accurate devise was used in order to measure it. Instead of using a ruler with divisions at every mm a ruler with divisions at every ½ mm would make the results even more accurate. The horizontal method of the experiment would also ensure that gravity does on act on the piece of fishing wire and the weight being applied will be distributed evenly across the fishing wire.
As further investigation the whole experiment would have to be done again using the horizontal method. Another possibility is that instead of testing one, or the same, material other materials could be tested. For example 32swg copper wire or nylon. This would give a broader understanding of materials which behave in accordance to Hooke’s Law. Different materials could also be compared so that it can be understood whether or not all materials behave in accordance to Hooke’s law. The materials that could be compared are copper wire, rubber and nylon fishing wire.
Apart from the material and the accuracy, there are other ways of conducting further work into Hooke’s law. The other two variables that could be changed are the thickness of the wire and the length of the wire to begin with - when it has no strain. The thickness would have to be just right so that the wire does not take too much of a load to snap but equally does on snap under a fairly lighted load.
Another change that could be made in order to gain detailed results is the Force which is being applied. Instead of applying 2N, as in this investigation, ½ N would mean that the results can be studied more carefully. This would also mean that the elastic limit can be pinpointed to ½ N instead of 2N.
