Understanding Why No Torque is Produced at O

[princejan7]

Homework Statement

http://postimg.org/image/tdocieaxx/

Why does the force system produce no torque about O
don't mg and N produce a torque about O?

Homework Equations

The Attempt at a Solution

 

[SammyS]

Homework Statement

http://postimg.org/image/tdocieaxx/

Why does the force system produce no torque about O
don't mg and N produce a torque about O?

Homework Equations

The Attempt at a Solution

How is torque about a point calculated?