Calculating Stresses at an Angle: Why Does it Matter?

[fonseh]

Homework Statement

Why can't i use the equation to determine the normal stress and shear stress ? Is my concept wrong ? I use the method in the link to do this question. The question said the
stress on a plane making angle 30 degree anticlockwise from plane AD .

Homework Equations

The Attempt at a Solution

When it's anticlockwise , the angle is positive
http://www.dummies.com/education/science/using-mohrs-circle-to-find-principal-stresses-and-angles/
Mod note: Image of work has been deleted. OP will be including work in a subsequent post.[/B]

 

[fonseh]

$$\sigma _(max) $$ = ((25-15 )/2 ) + 20cos60 + 20sin60 = 32.3MPa$$\sigma x_(min) $$ = ((25-15 )/2 ) - 20cos60 - 20sin60 = -22.3MPa

But the ans is 12.5 and -2.3MPa

 

[fonseh]

$r = sqrt ( ((\sigma _x -s \sigma_y) /2 )^2)+ (\tau_xy)^2) = 28$

$tan2\theta_p = 2\tau_xy / (\sigma _x -s \sigma_y) = 2(20) / 40$ , $\theta_p = 22.5$ degree ,
This is different from the $\theta_p$ = -68 degree given

 

[fonseh]

Image for mohr's circle
https://www.physicsforums.com/attachments/dsc_0038-jpg.110448/

 

[fonseh]

Is the ans provided wrong ? Or is my concept wrong ?

 

[fonseh]

bump