Physics Quiz

[BRAVESaj25bd8]

I'll use x,y,z to denote right direction, up direction, out of the page direction respectively, based on your drawing.

Sum of moments about the center of the wheel (z axis) shows that the y component of the friction force is tau/r.

Knowing that the axle is a two force member (cannot translate a moment), the friction force must be in the direction theta away from the x axis pointing up and to the right.

The magnitude of the force can be found by finding the resultant of the friction force from the y component we have already. Thus, the friction force is acting at a magnitude of tau/(rsin(theta)) at an angle theta above the horizontal (pointing up and to the right).

Solid question.