# Mechanics of Materials - Where's my mistake?

Gold Member
Aha, I knew it Thank Klaas, this helps confirm and I will show it to my teacher on Friday.

So, me and her set on yet another exercise and we got different results. We didn't make a bet this time...(she probably wants to, I'm unsure if I used the correct formula for area). The only problem is that this time my results are different than hers. So we got 3 different sets of results. My teacher's, mine, and her's. We would really appreciate your expert's opinion on this one (even delayed the control engineering exercise for this). Who do you think got it?

The problem:

http://img97.imageshack.us/img97/1213/exdia.jpg [Broken]

Solution one (hers):

http://img825.imageshack.us/img825/5094/darsol.jpg [Broken]

Solution two (mine):

http://img59.imageshack.us/img59/1221/orsolwhatswrong.jpg [Broken]

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I like Serena
Homework Helper
You both seem to have trouble with the calculation of area.

Your friend seems to have calculated the surface area of the inside of the rod, but that is empty air.

You seem to have treated the thickness of the wall as if it is a diameter on its own, which it isn't.

At top view of the rod would show 2 concentric circles.
The cross sectional area of the rod is the surface of the outer circle minus the surface of the inner circle.

Gold Member
I just calculated and u were right. Actually, I was rushing in with the area formula and I knew that was the problem. I should've just thought a bit and did it the long way instead of looking for shortcuts. Thank you very much ILS, you're the best.

I like Serena
Homework Helper
Hi friend of Fp!

I only got one minor comment.
Your graph of the length expansions has its zero at a.
But the zero should be at c, which is where the ground is, and where there will be no contraction.

Hi I am the friend, just to confirm:

A= π/4*(D)^2-π/4*(D-2α)^2

I like Serena
Homework Helper
Hi I am the friend, just to confirm:

A= π/4*(D)^2-π/4*(D-2α)^2

Yep. That is the proper formula for the area of the cross section.

Thanks I'll change that, but I am allowed to make those mistakes since I don't really study it, I just do it for fun :)

I like Serena
Homework Helper
Well, I'm just doing this for fun too.
But I don't allow myself many mistakes! ;)

Gold Member
Well, I do it for fun and for a degree! Thanks for helping out ILS we got it done, and are off to solve more complicated stuff now. Hopefully it'll go more smoothly.

I was trying to solve this exercise but since it's in English I wasn't sure I understood it.

this is what I did:

I was a little confused with +/- sings on F(bc).

I like Serena
Homework Helper
The signs appear to have been no problem at all and your work is neat!
I give it the ILSe Seal of Approval!