The force needed to extend a steel spring is proportional to its change in length. The change in length is referred to as the extension. The link between force and extension for a steel spring is known as Hooke’s Law.
I aim to show how much force (N) a spring, rubber band and polythene strip stretches before they reach their elastic limit; and whether the width of the materials can alter the results.
PLANNED METHOD
I am going to do a simple experiment that involves getting 3 different materials and hanging weights of different masses on them. Before I will do this I will measure the lengths of each material before hand. I will then attach a small weight to each material this will keep the material straight. I will then add more weights on the material in order to make it heavier.
Each time I add a weight I will record the new length of the material and the extension. I will need 6 different weight masses. I will then plot my measurements on a graph, as an extension (on the vertical axis) against the weight.
To make it a fair test I try to gently add the weight so that the force with which it landed does not alter the results. The most important factors are that I make sure that it is a fair test and that I take accurate measurements.
I will need to take measurements in millimetres after every 50g is added. The reason I should obtain accurate results is because I know that each material should stretch at equal amounts until they have been stretched beyond their elastic limit.
SAFETY
I will have to be careful to make sure that if the material breaks or snaps the falling weight could not be a danger to anyone. I will be taking minor precautions and shall not be adding ridiculous amounts of weight onto the materials. The measurements that I will be taking must be done as fairly as possible or the results shall not be accurate. The materials should be weight at the side of the table, so if it does snap it won’t harm anybody. Goggles are a necessary just in case the spring snaps and goes in the person’s eye. Since we will be working in pairs, I will have to keep my hand on the stand in order to prevent downfall of the equipment. The equipment that I will use should be checked to make sure that there is nothing wrong with it; I will make sure that I use the best equipment available to me.
Factors, which effect the stretching of a spring, are: -
· Downward force applied to spring.
· Spring material.
· Length of spring.
· No. Of coils in spring.
· Diameter of spring material.
· Cross sectional area of spring.
PREDICTION
I predict that the greater the weight applied to the spring, the further the spring will stretch. This is because extension is proportional to load and so if load increases so does extension and so stretching distance. The graph of the spring will be a straight vertical line on the y=x axis.
The rubber band will stretch, however only up to a certain limit (elastic limit). When reached its elastic limit it will break and snap. I predict that the graph of the rubber band will be straight and then will go upwards as the weight increases.
The Polythene strip will stretch only a little bit and will then loose its elastic limit. I predict that the graph of the polythene strip will be straight at first (horizontal) until it reaches its elastic limit. It will go up sharply, diagonally loosing its elastic limit as the weight increase.
I base this prediction on the secondary source below (Key Science Physics Jim Breithaupt).
Extension = New length – Original length
NUMBER AND RANGE
I will use 20 different weight masses such as 0.5N, 1N, 1.5N, 2N, 2.5N, 3N and so forth, all the way up to 10N. I will measure the materials and the extension in “mm”.
EQUPMENT
1. mm Ruler
2. Hanger
3. Steel Spring
4. Weights
5. Stand
DIAGRAM
OBTAINING EVIDENCE
Our results are as follows: -
SPRING 1ST TEST
SPRING 2ND TEST
Since we have 2 sets of results, I will find the average length, and then the average extension to make my results more accurate.
I will do this by adding the New Length of the 1st set of results to the new length in the 2nd set of results; and then diving it by 2, will give me the average length. The average extension will be simply taking away the average length away from the Original Length in the 1st set of results.
AVERAGED RESULTS
RUBBER BAND 1ST TEST
RUBERBAND 2ND TEST
AVERAGED RESULTS FOR RUBBER BAND
ELASTIC 1ST TEST
ELASTIC MATERIAL 2ND TEST
AVERAGED RESULTS FOR ELASTIC MATERIAL
ANALYSING EVIDENCE AND DRAWING CONCLUSIONS
From my results I can see that as load increases so does extension of a spring. This extension is measured in N/m (Newton’s per Metre)
The principle of Hooke’s Law can be seen in a graph of spring extension.
1. At this point the limit of proportionality has been reached this is where Hooke's law is no longer accurate.
2. This point is the spring’s elastic limit if the force is removed from the spring it will no longer return to its original shape.
Beyond this point the atoms in the spring material begin to break their bonds until eventually the spring yields and breaks.
This shows that Hooke’s Law does have a limit as a spring does and does have limitations and if kept within these boundaries will provide reliable accurate results. This is always the case; on the other hand I tested up to 10N for the spring.
Hooke’s law can also be explained by the molecular structure of the substance the force is acting on:
1. As molecules are pushed together, the larger the push the stronger the resistance.
2. At this point Hooke’s law of proportionality can be applied.
3. At this stage the effect of force becomes less until the spring yields and separates
(breaks).
The results I have gathered are reasonably reliable and accurate and as can be seen in the first experiment graph all of the results were very close to the line of best fit. The second experiment with the rubber band however, shows us that its elastic limit wasn’t achieved. The Elastic Material however achieved its elastic limit when it got to around about 3.5N.
In the test there were no significant anomalous results. In terms of reliability the experiment was carried out 2 times for each material, and then an average was taken out, from each of the materials tested.
In conclusion I am pleased with my results and feel they support each other as well as the laws they were based on. However if the experiment were to be carried out again changes could be made to reduce experimental error. A pointer on the spring would help in gathering the information more accurately. If the measurements were to be taken even more accurately an ultra sonic measuring device to measure the extension to a very accurate degree. I conclude to say that my prediction was correct on the basis that the spring obeys Hooke’s Law and that the Elastic material and rubber band does not do so.
