Help Needed with Final Question on Electrical Course

[ryanmcarthy]

Hi All,

I am really struggling with a question, and this is the last one I need to complete and then I can submit my paper.

I appreciate you don't like to just give out answers, but I can't work anything out. I am doing an electrical course, but for some reason there are some physics questions in, which I don't understand.

http://i570.photobucket.com/albums/ss141/mcarthyryan/Pg5.png

Thanks a lot

 

[berkeman]

Hi All,

I am really struggling with a question, and this is the last one I need to complete and then I can submit my paper.

I appreciate you don't like to just give out answers, but I can't work anything out. I am doing an electrical course, but for some reason there are some physics questions in, which I don't understand.

http://i570.photobucket.com/albums/ss141/mcarthyryan/Pg5.png

Thanks a lot

What are the relevant equations? What is the name of the class, and what year?

 

[ryanmcarthy]

Hi,
I'm not sure on the equations :( And the course is Electrical and Electronic Engineering, hence why I am struggling with physics questions.



Thanks a lot

 

[berkeman]

Hi,
I'm not sure on the equations :( And the course is Electrical and Electronic Engineering, hence why I am struggling with physics questions.



Thanks a lot

That is no help. Please re-read the PF Rules link at the top of the page, and try again...

 

[MATLABdude]

Do you have a textbook or some course notes to help you along? All of the posts you've posted seem to be of an introductory mechanics / statics bent, so I'm assuming you're working through a textbook / course of some sort? Like Jerry Maguire says, "Help us help you".

EDIT: If not, here's a starting point for part (a):
http://en.wikipedia.org/wiki/Buckling

 

[ryanmcarthy]

Hi Guys,

Apologise for lack of input in my previous post :)

I've had a go at the answers, and hopefully these are nearly there, could you possibly check them out?


a)

Euler buckling

Yield load Pc = σy A = π2 E I / leff2

Fixed ends
σy A = π2 E I / leff2 = 4 π2 E I / l2 or l2 = 4 π2 E I / σy A
I = π (do4 – di4) / 64,
where do = outer diameter = 80E-3 and di = inner diameter = 60E-3
and A = π r2, where r = (do – di ) / 2

So inserting all the data, the critical length lcr becomes 0.0131 m

b) i) Buckling mode shown in figure.
Can't draw this on here, but I think it is a column, a bit like "I", but with a bend in the middle?


ii) Critical load: Pcr = σy A = 308 kN

c) i) For a short column, the failure comes through compression rather than buckling.

ii) Johnsons formula gives the critical load as:

Pcr = σy A (1 - ) = 308 kN



Am I getting there? Thanks a lot

 

[ryanmcarthy]

Columns - Buckling and Failure

Hi all, Could some one clever check my answers please? I think and hope they are right, but any help would be great!

Question:

http://i570.photobucket.com/albums/ss141/mcarthyryan/Pg5-1.png

MY answers:

http://i570.photobucket.com/albums/ss141/mcarthyryan/Pg5Answers.png



Thanks a lot
Ryan

 

[SteamKing]

The calculation of the cross sectional area A is incorrect. There may be other problems in the calculation with using the right units.

If you think about the kind of structures which are prone to failure by buckling, long skinny members will buckle before short fat ones. Your effective length looks to be quite short for buckling failure, given the diameter of the pipe.

 

[ryanmcarthy]

Could you point out which calculation please?

 

[SteamKing]

The cross sectional area of the pipe, A = (pi/4)*(do^2-di^2) sq.mm Your calculations used r = (do - di) / 2 which is incorrect. The wall thickness of the pipe t = (do - di) / 2
It makes a big difference in the magnitude of the area.

 

[ryanmcarthy]

Thanks a lot for that, I will re-do them.