Torque ($\vec{\tau }$), or the moment of a force, is the tendency of a force to rotate a body about an axis. $\vec{\tau }=\vec{r}\times \vec{F}$ The magnitude is given by: $\tau =rF\sin \theta$ When the force is applied at a perpendicular distance ($r_{perp}=r$), the angle $\theta =90^{\circ }$ and $\sin 90^{\circ }=1$. Thus, $\tau =rF$.