Prediction
I predict that the sag will increase when I increase the load although the greater the sag becomes more weight will be needed to increase the sag the same distance. This is because gravity will pull down on the load putting the top of the cantilever under tension. This is when the molecules are stretched apart; the greater the stretch the greater the force of attraction The underneath of the cantilever will be put under compression which means the molecules are squeezed together. The closer they become the greater the force of repulsion. This means that a greater load is needed to separate the molecules further and increase the sag.
When the cantilever is stretched beyond its elastic limit, it will have a permanent change in molecular structure. Whole layers of molecule slide over one another; this is when the permanent change takes place.
Preliminary Experiments
To be able to carry my experiments out with precision I need to do some preliminary tests. This will help me to decide what the measurements of my fixed variables should be.
Firstly, I need to decide on how far the overhang of the cantilever should be.
Secondly, I have to decide what I should increase the weight by. I have decided that it would be best to have an overhang of 60cm because the cantilever had a permanent shape change with 70cm and the smallest weights would not have been measurable with the maximum weight allowed at 50cm.
I am going to increase my weights by 0.4N because the increase was enough to be able to record but wasn’t too much as to have few results.
Results
Analysis
The graph featured on page 4 shows the average results obtained from my experiment. I can conclude from this that cantilevers do have an elastic limit at which a permanent shape change takes place because the graph begins to curve at the top. I can also conclude that there is a positive correlation between the load of a cantilever and the sag which means that the more weight supported by a cantilever the bigger the sag will be.
A greater load is needed to stretch the molecules of the cantilever further apart and the further apart they become the greater the force of attraction; I can tell this because the sag increases less each time as the load increases. This is also true for the molecules on the bottom of the cantilever because the closer they become the greater the force of repulsion which will also lower the rate of increase in the sag.
Evaluation
I believe that I carried the experiment out with precision and care because I followed the safety aspects and precautions that I wrote about in the method and I also managed to obtain all the results in the time given.
My results are quite accurate because they nearly all fall on the line of best fit. There aren’t any really odd results so I am unable to comment on them. My results support my conclusions well because they are accurate.
For future improvements on the accuracy of my results I could have tighter control on my fixed variables. This would improve my experiment because it would mean that my results would be more accurate. Using the same set of weights for each repetition would also ensure the accuracy of my results. I could also obtain results with the overhang of the cantilever as my input variable and load as the fixed variable. This would also help to prove that my prediction is correct and accurate.
I could expand my experiment by using different types of cantilevers to see if they follow the same pattern as this cantilever and therefore I will have more evidence to back up my prediction.
By Stacey Robinson 10C1