Torque about the axle of a rotating disk

[kathyt.25]

Homework Statement

"The 19.8 cm diameter disk rotates on an axle through its center. F1=18.1 N, F2=26.6 N, F3=26.6 N, F4=18.1 N, and d=4.64 cm. What is the net torque about the axle? "

http://i4.photobucket.com/albums/y111/kathy_felldown/kn-pic1314.png

I have two questions about this problem:
(1) How do I know whether each force results in a negative or positive torque? I know that the torque conventions are positive if counterclockwise, and negative if clockwise, but the problem doesn't indicate the directions of the forces.
(2) Which forces result in no torque? I learned that if a force is parallel or acting on the axis (in this case, there are no forces acting directly on the axis), that force doesn't produce any torque. How do I denote which force is acting parallel to the axis, since its a disk?

Homework Equations

The Attempt at a Solution

 

[kathyt.25]

Forces acting parallel to the axis don't produce torque?

Homework Statement

I learned that if a force is acting parallel, or acting on the axis of rotation, it doesn't produce torque. If a force is acting at an angle to the axis, does that mean that only ONE component of that force (ie. the y or x component) actually produces torque?

How do I know which one is the force component that doesn't produce torque, if it's not a straightforward situation like an opening door, and instead, a more ambiguous situation like a rotating disk?

Homework Equations

The Attempt at a Solution

 

[berkeman]

Homework Statement

I learned that if a force is acting parallel, or acting on the axis of rotation, it doesn't produce torque. If a force is acting at an angle to the axis, does that mean that only ONE component of that force (ie. the y or x component) actually produces torque?

How do I know which one is the force component that doesn't produce torque, if it's not a straightforward situation like an opening door, and instead, a more ambiguous situation like a rotating disk?

Homework Equations

The Attempt at a Solution

That's why you use the vector form of the equation for calculating the torque. Can you show us that vector equation?

Torque = ??

 

[berkeman]

(I merged your two threads asking a common question)

Also, we would need to see the drawing from the problem to help you with the question on which forces do what.