Variable to change: weight each time only. Keep the material of the cord the same each time. If the material of the cord was changed half way through the experiment, this would not produce an accurate set of results as the material of the cord could be stronger or weaker than before and this would affect how it would react to weight on the end. If the material is weaker, then the cord would snap sooner or extend more than the cord may have done before. If the material of the cord is stronger than before, then it would not be affected as much by the weight on the end and may not as extend as much as the cord did before it was changed half way through the experiment.
Keep the length of the cord the same each time when putting weight on the end. If the length of the cord was changed each time then an accurate set of results will not be produced. If the length of the cord was shortened then obviously the reading of the extension will get less each time. We will not properly see the extension of the cord; we will instead get a confusing set of results as we would not have investigated the extension of one piece of cord. Also keep the thickness of the cord the same or this will not produce an accurate set of results as the thicker the cord, the less the extension will be. The cross-sectional area and the diameter of the cord will also affect the extension.
Do not throw the weight down; just let the cord drop down. If we drop the weight then it will take a while to stop bouncing and we will not be able to take an accurate reading from the bottom of the weight. Let the weight go from the same height above normal length. Drop the weight from rest so that it does not bounce so much and we can therefore take a fairly accurate reading.
The speed the weight is dropped from will affect the extension but this is hard and we cannot measure the speed that we drop the weight.
The temperature of the cord will also affect the extension but we cannot vary this nor do anything about it.
Air resistance is another factor that will affect this experiment, but there is nothing that we can do about air resistance.
To make sure that this is a safe test, we will act sensibly and we will give ourselves enough space to work in. We will not throw down the mass as it may bounce up again and cause someone an injury. No strict safety precautions need to be used, as the only potential danger would be if the cord snapped, however this will not happen if there is no more than the maximum load on the cord.
Equipment needed: Tape measure
Rubber cord
100g of weights
Clamp+ clamp stand
2 2p pieces (to clamp the ends of the cord between).
Set the equipment up as shown in the diagram.
*Double the cord up so that it is stronger and clamp the ends between the 2p pieces which is held between the clamp. Hook the 10g weight holder *
First measure the natural length of the cord from the clamp to the end. We must make sure that we have no paradox, we must measure the extension from the bottom of the cord so we must be eye-level with the cord at the bottom to make sure that our reading is accurate. Each length will be repeated 3 times and an average will be taken to ensure an accurate test. The mass on the end of the cord will be changed by 10g each time, starting from 10g and working our way up gradually to 100g. The extension is l2-l1. L1 is the original length of the cord without any weight on the end, and l2 is the extension each time after a weight is added on the end. Extension = New length – Original length.
We need to produce precise and reliable evidence so we can obtain fair and accurate results to produce a good graph with noticeable patterns and a line of best fit with hopefully no anomalous results. We can then spot trends and produce a detailed and correct conclusion. We will need to make sure that our experiment runs smoothly and there are no complications by following the guidelines above carefully about making the experiment fair and accurate. We need to produce precise evidence (for example 10.00g instead of 10g) because then we do not have the risk of rounding up decimal places wrong, and ending up with the complete wrong number which could produce anomalous results on graphs and this evidence may not be accurate enough to draw a firm conclusion from. It may mess up trends on graphs and in results.
We need to produce reliable results each time so we can draw a correct and accurate graph and identify trends in the graph and our results. We can therefore come to a firm conclusion using our results obtained. So, replicates are essential for all measurements, i.e. getting the same answer at least 2 times out of 3 after repeating each measurement. To do this, we must read measuring instruments carefully each time.
Carry out the experiment, dropping the mass from rest, and working your way up from 10g to 100g, increasing by 10g each time. Repeat each weight 3 times and take an average from those measurements you obtain. Using the tape measure, measure from the top of the cord clamped between the 2p pieces to the bottom of the cord where the mass hanger with weights on is hung at the end. If a measurement is taken from the top of the clamp to the bottom of the weight on the end, we are not actually measuring the extension of the cord.
Results
Results table to show how the weight hung on the end of a piece of rubber cord affects the extension of the cord.
Please see separate results graph.
The graph is plotted using the weight (g) and extension of cord (cm).
The results show us that as the weight hung on the end of the cord increases, so does the extension of the cord. The results shown in this table prove my prediction true; I predicted that the greater the weight applied to the cord, the further the cord will stretch. This is because of Hooke’s law: extension of a cord or spring is proportional to the downward force acting on the spring or cord. The force a cord exerts on a body is directly proportional to the displacement of the system (extension of the cord).
The results proved my prediction to be true because they showed that the extension of the cord increased when more weight was hung on the end.
The graph was also accurate and a line of best fit could be drawn, showing that there is a noticeable pattern in my results; the greater the weight applied to the cord, the further the cord stretched. At the end of the graph, the results seem to tail off at the end. This could be because the cord is reaching its yield point and will snap very soon if any more weight is added onto the end of it.
Conclusion
From our experiment, we can see that, due to Hooke’s Law, the more weight that is hung on the end of a piece of rubber cord, the greater the extension of the cord will be. This is because the extension of a cord or spring is proportional to the downward force acting on the spring or cord, so the more weight hung on the end of the cord, the greater the force acting downward on the cord. This will therefore increase the extension of the cord. We can see from the graph that this has been proved true, as the graph shows a clear curve indicating that the extension of the cord has increased when more weight has been added onto the end. The positive gradient of the graph shows us that as the weight on the end increases, so does the extension of the cord.
Evaluation
I think our experiment went quite well, we produced a fairly good set of results that proved the prediction I made to be true. A good graph could be drawn up and a line of best fit could be easily drawn. When we re-measured each measurement, the results were recorded were all similar. This shows an accurate and reliable test. The procedure used was accurate as a good set of results were produced. We can tell they were accurate because they fit in with the information I found out about Hooke’s Law and these results also proved my prediction made that was based on Hooke’s Law true. There was only one extremely slight anomalous point that can be seen on the graph, at 100g. This is however, not too far away from the line of best fit but it could have been obtained due to slightly unreliable and inaccurate measuring. The results we have recorded in the graph are not entirely accurate, to have an accurate enough experiment, the measurements need to all be similar, but ours were not entirely so. Sometimes there was a difference of 4cm between the 1st measurement and the repeat. We tried to make sure that we had no paradox when recording measurements but humans aren’t the most reliable and accurate measurers in the world. To improve measuring methods, if we could have access to them we could use lasers to gain an incredibly accurate measurement from the tape measure. The procedure was suitable to the investigation as we did obtain all the information that we were aiming to investigate. The evidence we gained is sufficient for a firm conclusion because, as I have said previously, the results fit in with the information I found out about Hooke’s Law.
Some further experiments that can be carried out to extend the enquiry could be done changing the other variables that I have mentioned before:
- the material of the cord
- the length of the cord
- the cross-sectional area and the diameter of the cord
- speed the weight is dropped from if we could find out a way of measuring this
and
- temperature of the cord if we could also find out some way of measuring and varying this.
