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# How to find extension of cord using young's modulus

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1. How do you find the extension of a cord once given the young's modulus, force, area and UPSTRETCHED cable length? Therefore we are given only extension + original length i.e L + change in L, which does not fit into any of the equations as we have 2 unknowns?
2. (Original post by mayjb)
How do you find the extension of a cord once given the young's modulus, force, area and UPSTRETCHED cable length? Therefore we are given only extension + original length i.e L + change in L, which does not fit into any of the equations as we have 2 unknowns?
You'll have to rejig the equations.

Starting with the definition of Young's modulus:

Where is the unstretched length and the extension.

If L is the upstretched (sic) length, we can write

Sub into our original equation to get:

Now make the subject - left as exercise.

And hence work out

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