The spool shown in the figure is placed on a rough horizontal surface and has an inner radius $r$ and an outer radius $R$. The angle $\theta$ between the applied force and the horizontal can be varied. The critical angle $\theta$ for which the spool does not roll and remains stationary is given by:

  • A
    $\theta = \cos^{-1} \left( \frac{r}{R} \right)$
  • B
    $\theta = \cos^{-1} \left( \frac{2r}{R} \right)$
  • C
    $\theta = \cos^{-1} \sqrt{\frac{r}{R}}$
  • D
    $\theta = \sin^{-1} \left( \frac{r}{R} \right)$

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