Salters Hornes CWK Young Modulus of a scooby

[thermoluminesce]

My cwk is investigating the youngs modulus of a scooby. A scooby is a hollow plastic string like thing. My experiment used the pulley clamped to the bench method and attaching weights.

I have my results and have worked out the change in area for each load and have a graph of stesslog10 vs strain.

I'm a bit lost as to what to do next though (I've lost my cwk guide thing). I'm a bit stuck on what error bars i should do, i thought I could have made an error of 0.005m on measuring the extension and so on the x-axis of my graph an error bar of +-0.005. But what for stress?

What do i need to conclude? How do I tell if my sample obeyed hookes law?

And then do i evaluate my method? I read in http://www.thestudentroom.co.uk/showthread.php?t=86576" Morbo's suggestion for a more accurate experiment, but I don't quite understand the reasoning for it (something about temperature change) or how exactly its done.

If anyone knows of any websites with cwk guides or tips or anything that would be much appreciated.

Thanks

 

[BigJDubs]

in advance :)Error bars are used to indicate the amount of uncertainty around a given measurement. To calculate the error bars for stress, you need to calculate the uncertainty in the measurements of force and area. The uncertainty in the force can be calculated by looking at the uncertainty in the weights used. For example, if the weights are given to the nearest gram, then the uncertainty in the force would be 0.001 Newton. The uncertainty in the area can be calculated by looking at the uncertainty in the measurements of the length of the scooby. If the measurement is taken to the nearest millimeter, then the uncertainty in the area would be 0.01 square meter. Once you have these uncertainties, you can then calculate the uncertainty in the stress using the formula Stress = Force/Area. The error bar for the stress would then be calculated using the formula Error Bar = Uncertainty in Stress/Stress. To determine if the sample obeyed Hooke's law, you need to compare the graph of stress vs strain with a graph of the predicted values according to Hooke's law. If the results match closely, then the sample obeyed Hooke's law.The suggestion Morbo made was to use a strain gauge to measure the strain more accurately. This is because a strain gauge is more sensitive to temperature changes than a ruler, which would be less accurate when measuring the strain. The strain gauge works by measuring the electrical resistance of a wire when it is stretched, so the strain can be calculated from the change in resistance.