Simple Mohr's Circle Question - Axis scales?

[Roomie]

I'm just wondering about using a Mohr's Circle, do I need to use the same scale for the x and y axes, as surely otherwise my choice of scale greatly impacts the results.

I am using one to get my principle second moment of areas, Ix and Iy, for an equal L section.

Ix and Iy are where the Mohr's circle cross the x axis, but depending on my scale this can vary hugely!

I'm just wondering if anyone has any advice?

Thanks!

Note: This is when the two points on my Mohr's circle have the same value on the x-axis, so when I join them up I have a vertical line.

 

[Roomie]

I realize this was slightly confusing but this is what I mean:

Here are 2 different Mohr's Circles, that cross the x-axis at different places just based on the scales.

So which is correct?

http://dl.dropbox.com/u/630750/Screen%20Shot%202012-01-04%20at%2021.48.12.png

http://dl.dropbox.com/u/630750/Screen%20Shot%202012-01-04%20at%2021.48.44.png

 

[nvn]

Roomie: The x and y axes should have the same scale. I think neither of your Mohr's circles currently look correct. (1) Put the centre of Mohr's circle at (x, y) = [0.5(Ix + Iy), 0]. (2) Plot point A at (x, y) = (Ix, -Ixy), which is a point on Mohr's circle. (3) Draw the circle passing through point A. (4) Now you can see the points where Mohr's circle crosses the x axis, which are I1 and I2.