# Direct stress and strain Q

1. Dec 6, 2013

### anthonyk2013

A light alloy bar is observed to increase in length by .35% when subjected to a tenstile stress of 280MN/m^2.
Calculate youngs modulus for the material.

σ=280MN/M^2
x=.35%
ε=?
E=?
L=?

I have too many unknowns where do I start?

2. Dec 6, 2013

### SteamKing

Staff Emeritus
What's the definition of Young's modulus? How is it calculated?

3. Dec 6, 2013

### anthonyk2013

E=σ/ε

to find ε=x/L so think I might have to work of .35% but not sure how.

4. Dec 6, 2013

### SteamKing

Staff Emeritus
What does the 0.35% represent?

5. Dec 6, 2013

### anthonyk2013

The length of the alloy has increased by .35%

Last edited: Dec 6, 2013
6. Dec 6, 2013

### SteamKing

Staff Emeritus
And how would this increase be related to the strain?

7. Dec 6, 2013

### anthonyk2013

Is strain not increase in length/original length ?

8. Dec 6, 2013

### SteamKing

Staff Emeritus
Yes.

9. Dec 6, 2013

### anthonyk2013

I have the increase in length I don't have strain and I don't have original length.
Is it possible to work out length using only .35%?

10. Dec 6, 2013

### SteamKing

Staff Emeritus
Well, let's start with the basics. Assume the original length of the test specimen was L. After the stress was applied, what is the length of the test specimen? What is the strain due to the applied stress?

11. Dec 6, 2013

### anthonyk2013

Strain=.35/L

12. Dec 6, 2013

### SteamKing

Staff Emeritus
From the OP:

"A light alloy bar is observed to increase in length by .35% when subjected to a tenstile stress of 280MN/m^2."

Would you like to revise your answer?

13. Dec 6, 2013

### anthonyk2013

Sorry SK but I'm lost now.

14. Dec 6, 2013

### anthonyk2013

Stress and strain are proportional according to hooks law?

15. Dec 6, 2013

### SteamKing

Staff Emeritus
Yes, but you are avoiding the question about the strain of the test piece after it has the stress applied.

If the length of the test piece increases by 0.35%, what is its new length in terms of its original length?

16. Dec 6, 2013

### anthonyk2013

Sorry SK but I'm still lost. Do I need to convert .35 from a % to measurement by dividing by 100.

17. Dec 6, 2013

### SteamKing

Staff Emeritus
Look, if something increases in length by 25%, what is its new length compared to its original length?

18. Dec 6, 2013

### Staff: Mentor

If you have D dollars in your account to start with, and you earn 0.35% per year interest (compounded annually), how much money do you have in your account after 1 year? How much has the account value increased?

19. Dec 6, 2013

### anthonyk2013

D+.35%?

20. Dec 6, 2013

### Staff: Mentor

.35% is not an amount of money. To verify this, try spending .35% at your local store.

The amount of money you would have after 1 year would be (D+0.0035D), and the amount of money you would have earned would be 0.0035D dollars.

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