Stretching Materials

Planning My Work

For this experiment I will need to investigate the factors that effect the ‘stretch’ or extension of a piece of material.

Aim: To find a relationship between the length a piece of material can stretch under a certain weight, and how a certain length effects how much it stretches.

The extension of the materials I use will rely on whether the molecules are strong enough to hold a definite shape.

In a solid there are two forces acting. There are attractive forces if molecules try to move apart and repulsive forces if molecules try to move closer. If the material is stretched then the forces are unbalanced. This is what is taking place in our experiment.

We will want to investigate the stretch of our materials and so it is important to bare in mind this information to identify the key factors that might affect the extension of the material.

From this information we can predict that the weight we apply to our material will effect its extension by adding to the downward force acting on the material. However the properties of the material we use will also vary its extension by the individual structure of the molecules in each material.

To reach our aim we will also carry out a set of experiments, again with the different materials, where the length of the material is varied. We will carry out this experiment to reach our aim of investigating a relationship between weight and length. With a set of results showing how weight varies the extension we will be able to see if the length has anything in common.

My Experiment

For the experiment I have decided to use the materials of elastic and string to test their different properties and look for similarities and differences in the two materials.

Apparatus:

Stand, Scissors,

Boss, 10 Newton weights

Clamp, Metre ruler,

G-clamp, cellotape

Diagram:

Method:

Experiment One. Variable = Weight

For each of the materials we use we will carry out the same experiment.

We will set up the apparatus as shown in the diagram. Keeping the length of our material exactly the same. We will vary the amount of weight in newtons to test the properties of the material, following these steps.

Set up all apparatus
Secure a length of material 15cm long to the clamp.
Measure the length of the material with no weights on it
Attach the weight holder and the correct amount of weight.
Measure the extension of the material each time.

For this experiment we will need a range of weights, in order to collect the information we need. We decided to start are range at 1N and add a Newton each time until reaching 10N. It is also important to include a measurement for 0N to show the original length of the material.

I chose the length of 15cm for our material, as it is sensible and easy to work with.

Once the experiment has been carried out, then it should be repeated with the other material.

Experiment Two. Variable = length

Again we will repeat this experiment ...

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I chose the length of 15cm for our material, as it is sensible and easy to work with.

Once the experiment has been carried out, then it should be repeated with the other material.

Experiment Two. Variable = length

Again we will repeat this experiment for each of the materials.

Set up all apparatus
Secure the correct length of material to the clamp.
Measure the length of the piece of material with no weights on it
Add a weight of 5N to the material and measure the extension
Repeat this for the different lengths of material.

The weight will stay the same through so that there is only one variable. For the variable of length we chose a range beginning at 2cm, adding 2cm to this length each time until the material is a length of 20cm.

For both the experiments I chose the ranges to give us accurate results by keeping the readings close in order to see the most precise readings as possible. By using these, it should be possible to see any patterns in the results.

It is also important to verify that for each experiment, the total length of the material, minus the original length measures the extension. This can more simply be carried out by beginning the measurements from the bottom of the original length and measuring how much it goes past this point when extended.

Fair Test:

It is very important to make my experiment fair. If it is not kept exact, then my results may easily be altered by slight differences that shouldn’t occur.

In order to keep my experiment fair, I will ensure that there is only one variable at any time. This will be either the weight or the length. Other possible variables such as the amount of strands of a material or the type of material should never change and should always stay the same unless specified. The other variable out of length or weight, which is not the focus of the experiment at the time, should also not alter, unless it is specifically part of the experiment.

If this is kept to then my experiment will be fair, hopefully giving me the most accurate results as possible.

Safe Test:

Keeping a safe test is also very important for the safety of me and other people around me. To ensure that my experiment is safe, I will carry out the following precautions.

Goggles should be warn to protect my eyes from stray objects or in case an accident occurred
The G-clamp should firmly clamp the apparatus to stop it from toppling over and causing damage
I should act in a sensible and safe manner, carefully working with the equipment and being aware of any dangers.

Obtaining My Evidence

The next stage was to carry out my experiment, following all previous planning precisely. By doing this, I obtained the following results.

Experiment One. Variable = Weight

Experiment Two. Variable = length

I have put my results into these tables to show them clearly. I am then able to easily compare the data and see and obvious trends. My results are then a lot easier to handle.

Analysing My Evidence

With the data I had collected, I decided to create scatter graphs to show even more clearly the information. To my points, I added a line of best fit in order to show the trends and patterns. I did two separate graphs for each experiment as they were holding deferent information, but showed the results for the string and elastic on the graphs together to show how they compare to each other. These graphs can be seen on the next pages on the graph paper.

I am first going to look at the graph for experiment one where the variation is the weight. We can see that there is a clear pattern of increase in the extension.

We can see that with the elastic, at first there is a sharp increase where the extension is directly proportional to the stretching force, and is shown by a straight line through the graph. This means that the same amount of weight added is equal to the same extension of the material, and is called Hooke’s Law. In this experiment, up to 2N, the elastic gives an equal extension to an equal weight and obeys Hooke’s Law. The 2N point is the elastic limit, and past this point the elastic acted differently and did not obey Hooke’s law. I labelled these additional notes on the graph.

Looking at how the string extended in experiment one, we see how, especially in comparison to the elastic, it extended only a very minute amount, and almost not at all. This already shows us how string and elastic react very differently and therefor have different properties.

In order to compare all information, I looked at the graph produced by the second experiment. Again, the general trend for both the elastic and string was of an increase and it was also very noticeable how the string produced very similar results to in experiment one. Its extension was almost unnoticeable as it hardly increased. We could compare this to the line given by the elastic that again had a much larger increase, similar to the first experiment. However, we see no precise trend that the information follows and the points are slightly more randomly placed.

These are observations of the trends and patterns of my results that I can see on the following graphs.

I related this information back to my aim of finding a relationship between how the length of a piece of material can extend under a certain weight (experiment 1) and how a certain length effects how much it stretches (experiment 2). In order to do this, I needed to look at the differences and similarities between the two graphs representing both experiments, and look for any direct, or indirect relationships.

What is most noticeably in common is the increase of extension in both materials for experiment one and two. We see that the more weight that is added, and the longer the material is, the more it extends.

In order to explain this, I have decided to use elastic as an example as it shows it the most clearly. It is the molecular structure of the elastic that gives it its ‘stretch’. In a larger scale, the molecular structure of elastic can be represented as a bundle of rope.

Because these are not tightly bonded together, if pulled, the strands can straighten out, extending their length.

The larger the force that pulls the elastic, the more the strands are straightened out. This explains to us how experiment one produced its results. As we increased the weight, we saw how the extension also increased. This is due to the strands being straightened and explains the section where Hooke’s law was obeyed. The elastic limit shows us where the stands are unable to be straightened any more and so began to weaken instead.

In order to explain experiment two, how the strands stretch need to be looked into in more detail. We need to see that the ratios of length to extension will always be concise in the same piece of elastic where the molecular structure is the same. For example, if a 5cm piece of elastic triples in size, it will have a 10cm extension. In a 10cm piece of elastic triples in size, it will have a 20cm extension. This trend will continue and you will be able to see an increase in the length of the extension.

This is what we saw happening in experiment 2. In order to find out how much the material was extending each time, I decided to draw out another table to show by how much a length was multiplied by to reach its extended size.

From the table we can see that our piece of elastic extended roughly double. Most of the above were around 1, higher or lower. As they are not particularly exact, we can presume that this may be due to small inaccuracies when carrying out the experiment.

This information concerning how a material will stretch, helps us to understand why the string and elastic reacted so differently to the same experiment. We need to go back to the bundle of rope theory and the elastic limit. The elastic limit is when the rope (molecule strands) can no longer straighten out any further and are tight along side each other. It is here that the elastic has the same properties as the string. We are then able to state that the reason the string extends so little is because its strands are already fully straightened and so are unable to extend any further. The elastic’s strands however are still completely un-straightened and so are able to extend a lot further.

In conclusion, an indirect relationship exists between how the length of a piece of material can stretch under a certain weight, and how a certain length effects how much it stretches as they both have an increase in extension due to the molecular structure of the material. However, there is not direct relationship as there is no link between the two factors.

Evaluating My Evidence

Looking back on the experiment I have carried out successful, dependable procedure that has given me precise and reliable evidence. Looking at the graph for experiment one, you can see that the points follow an almost exact line showing that the experiment was carried out accurately.

However, it is also interesting to see the graph for experiment two, and how many points on the graph do not follow the line of best fit and the points are slightly more randomly placed. It shows how this experiment produced less correct results to the first. This indicates that perhaps the experiment needed to be carried out more accurately as there at least two worryingly out of place, anomalous results.

These points would need to be repeated. Seeing as it was the second experiment only that needed more accuracy, I looked into different ways of improving the results. When we were previously carrying out the experiment, we had attached the material by tying it to each end. This perhaps was unwise and it is easy to imaging that the more of the material than wise is used up in the knot, rather than as the length. Creating loops may improve the results to give them a more regular pattern and make them more reliable.

It is obvious however, that even with these stray results, they can still successfully support a firm conclusion, and would be even more supporting is the few odd readings were repeated. I could also add additional evidence to the conclusion and extend the inquiry by looking more closely at the molecular structure of additional materials to widen my range of information.