# Homework Help: Youngs Modulus & Spring Constant Experiment

1. Dec 13, 2012

### kingstar

1. The problem statement, all variables and given/known data
I've been asked to find the stiffness constant of a rubber band and young's modulus.
To do this I created the experiment like this http://www.gcsesciencemethods.co.uk/2012/05/hookes-law-1.html and then i put all the values in a table. I plotted a graph of extension vs weight (i was asked), and found the spring constant.

The examiner doesn't really care about the value's its more to demonstrate my understanding.

Now i have a table with the:
Weight, original length, length (with weight added), extension.
My question is how do i find Youngs Modulus from this?

(I know the area, 2.93x10^-3m^2)
2. Relevant equations
F = -kx

YM = Stress/Strain

3. The attempt at a solution
I attempted to find the stress and strain at each weight.
Example: For one of the extensions i used a weight of 0.1N, so i used that data to find Youngs Modulus, but when i did it for 0.2N it wasn't the same.
This has left me so confused! :<

2. Dec 13, 2012

### kingstar

anyone?! :(

3. Dec 13, 2012

### PhanthomJay

Within the proportional limit and accuracy of the lab data (and applicabilty of Hooke's law for the spring), you should get a straight line relationship of force versus displacement, where the slope of the graphed line is the spring constant, k. In calculating strain, where strain is the displacement divided by the original length, you can do a bit of algebraic manipulation of your stress-starin equation to to conclude that k = AE/L, where L is the original length of the spring, not the stretched length.

4. Dec 14, 2012

### kingstar

Thank you. I've got it.