A wheel of radius R and mass M is placed at the bottom of a fixed step of height R as shown. A constant force is applied on the surface of the wheel so that it just climb the step without slipping then

If the force is applied normal to the circumference at point ‘X’ then $\mathrm{\tau }$ is constant 

If the force is applied to the circumference at point P then $\mathrm{\tau }=0$

If the force is applied tangentially at point ‘S’ then $\mathrm{\tau }\ne 0$ but the whell never climbe the step 

If the force is applied at ‘P’ 6angentially, then $\mathrm{\tau }$ decreases continuously as the wheel climbs