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# Elastic Strain Energy? Watch

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1. Is the equation for elastic strain energy (0.5 x F x deltaX) only for when hookes law is obeyed or can it be used when the graph goes curved (elastic/plastic region) as well??
2. I've just been revising this and if what I understand is correct it is only for the part where hooke's law is true, so in other words to find the elastic energy you are finding the area under the line which is 1/2(Force)(Extenision) and if you don't have F but you have K you can used 1/2 KE^2 where K is a constant. Finally if you have stress and strain you can work out the strain energy by doing 1/2 * Stress * Strain.
3. Then which equation should be used if the line is curved?
4. (Original post by Vampire-Love4ever)
Then which equation should be used if the line is curved?
It's still equal to the area under the (F-e) graph, even if the line is a curve.
When it's a straight line you have a triangle whose area is half base x height.
That's why the equation is then simply

You would need to know the equation of the curve and integrate it, as the poster above has said, if you wanted to calculate it directly from a "formula".
5. (Original post by Stonebridge)
It's still equal to the area under the (F-e) graph, even if the line is a curve.
When it's a straight line you have a triangle whose area is half base x height.
That's why the equation is then simply

You would need to know the equation of the curve and integrate it, as the poster above has said, if you wanted to calculate it directly from a &quot;formula&quot;.
We can also count the square boxes if we are asked to find Elastic Strain energy of a curved line graph. Right?
6. (Original post by Vampire-Love4ever)
We can also count the square boxes if we are asked to find Elastic Strain energy of a curved line graph. Right?
Yes, of course. You just need to estimate a little because the line will pass through squares. The usual trick is to count the (small) square if more than half of it is under the line, and ignore it if less than half. It will even out on average.
Also make sure you are clear what (energy) one square is equal to by checking, and multiplying together, the number of units each side is equal to.

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Updated: December 29, 2010
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