Solving a Wheel on Step Question - Torque Calculation and Solution Attempt

[TBBTs]

Homework Statement

Question is here. http://i.imgur.com/WOy0w.png

Homework Equations

Torque=FR

The Attempt at a Solution

1)
1)I thought it is radius. But radius is not one of the option
2)I thought this is radius too.
3)Not sure about this question,maybe get torque get acceleration and than use Mgr=I*alpha
Thanks!

 

[Undoubtedly0]

Greetings! Remember that torque is $$\tau = \overline{r} \times \overline{F}$$, which is different than $$\tau = rF$$. In other words, torque is the radius at which a force acts perpendicularly to the lever arm, times the magnitude of the force. Thus, for part A, note that the corner of the step will be exerting a force $$F_{min}$$ directly to the left in order to counteract the rightward force on the wheel's axle. If we extend this leftward $$F_{min}$$, we see that it acts perpendicularly to the wheel at a radius of $$R - h$$ (the moment arm is conveniently indicated by the vertical dotted line).

Therefore, for part A, $$\tau = \overline{r} \times \overline{F} = (R-h)F_{min}$$. Try applying this same strategy to part B.