Chapter 11, Problem 4Q

The position vector $\stackrel{\to }{r}$ of a particle relative to a certain point has a magnitude of 3 m, and the force $\stackrel{\to }{F}$ on the particle has a magnitude of 4 N. What is the angle between the directions of $\stackrel{\to }{r}$ and $\stackrel{\to }{F}$ if the magnitude of the associated torque equals (a) zero and (b) 12 N · m?