Young's modulus, stretching question.

[LordPride]

Homework Statement

Three separate members of steel, copper and brass are of identical dimensions and are
equally loaded axially.Young’s moduli for the materials are:
steel, 210,000 N/mm^2
copper, 100,000 N/mm^2
brass, 95,000 N/mm^2
If the steel member stretches 0.13mm,
calculate the amount of elongation in the copper and brass members.

Homework Equations

The equation i believe to be relevant;
Young's modulus=Stress/Strain

The Attempt at a Solution

I am confused how to approach this question at first i believed i had to find the stress by using the elongation of 0.13mm, and because all the members would be under the same stress i could then input that information into a youngs modulus equation for the other two members. However the young's modulus equation requires strain not just the elongation and i don't know the original length.

The only way I can think of solving this question would to assume that the strain = 0.13 and solve the question like so

Steel young's modulus= stress/strain
210=stress/0.13
stress=210*0.13=27.3

Starting with brass as an example
Young's modulus = stress/strain
strain= stress/young modulus
Strain= 27.3/95 =0.29 mm

and Copper =0.27mm

Is this assumption correct, if not any hints on how i could go about solving this problem?
Thank you very much.

 

[Doc Al]

However the young's modulus equation requires strain not just the elongation and i don't know the original length.

You don't need to know the original length, just that it is the same for each. Accordingly, the elongation will be proportional to the strain.

 

[LordPride]

Oh haha, thank you very much. My misunderstanding =D