# Young's modulus, stretching question.

## 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.

## Answers and Replies

Doc Al
Mentor
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.

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