# Homework Help: Young's Modulus - estimate elastic limit

1. Feb 20, 2010

### ZedCar

1. The problem statement, all variables and given/known data

I'm completing a homework for Young's Modulus and one question asks to estimate the elastic limit for the wire which was used.

I can see where this approximately is on the graph I have drawn. When the question asks to estimate the elastic limit, does this mean to simply state the mass which was applied when the elastic limit was reached?

Thanks

2. Feb 20, 2010

### ZedCar

I had another think about it. Would I be right in thinking the elastic limit would be the mass where this happens (in kg) multiplied by gravitational acceleration and the result of this multiplication stated in Newtons?

3. Feb 20, 2010

### PhanthomJay

No, the elastic limit is the stress at yield (yield stress of the wire, in N/m^2, or Pascals), not the force at yield. What are you plotting on your graph?

4. Feb 20, 2010

### ZedCar

Thanks PhanthomJay, it's mass / grams on the y-axis and extension / mm on the x-axis.

5. Feb 20, 2010

### PhanthomJay

Looks then like your plotting mass (presumably a mass hanging down on the wire, yielding a weight/force unit) versus extension.. You want to find the stress at yield. What is it? Are the properties of the wire given?

6. Feb 21, 2010

### ZedCar

Yes, that's right.

Ah, I see what you mean! I need to find the stress at the yield point.

So I'll obtain the applied force by using the mass (in kg) at which the elastic limit is reached and multiply this by 9.81 (acceleration due to gravity). Then divide this number by the cross sectional area of the wire. Ensuring when calculating the cross sectional area of the wire the diameter is in the SI unit of the metre.

So the final figure for the estimation of the elastic limit would be in the units of N/m2

7. Feb 21, 2010

### PhanthomJay

Yes, excellent. And i'm sure you know that a N/m^2 is called a 'pascal' for short.

8. Feb 21, 2010

### ZedCar

Yes, thanks very much!