Shear stress problem: What decides the sign of Tau_V?

[salamikorv]

Homework Statement

What decides Tau_V sign?

Relevant Equations

Tau_T-Tau_V

Im so confused on these solutions here for the shear stresses. Why is Tau_V sometimes negative and sometimes positive? Can someone please explain this and maybe illustrate?
Here for example.

Heres the solution for problem b):

Here Tau_V (talking about F_xQ/(Ir) ) is negative, so how im thinking why it is negative is because the moment is making so that the shear force on point P is directed to the negative x-direction, but on the drawing its directed to the positive x-direction??

And for point Q, Tau_V is positive? Why is it not negative? I thought since the bending onto the cross section is going clockwise, making the shear force at Q go at positive z-direction, it should be positive? But its wrong and how should i think about this?

 

[erobz]

Could you be more clear about your force , moment breakdown. Try to draw something 3d like I did for this problem

Yours is an even messier problem, and it's too hard to decipher your handwritten notes.

Also, please learn to use Latex to provide your equations. It doesn't take much to learn, and it helps people trying to help you. Simply put "Help us to help you". I know...organizing your thought takes effort. Furthermore, when you make a good diagram for a problem, sometimes you will clear issues up on your own. This was made with PowerPoint, but a good hand drawing will work too.

 

[Lnewqban]

Are you still interested in the solution of this problem, @salamikorv ?