# Magnitude of moment in 3d space

What's the magnitude of moment about O, if the vectors F, r1 and r2 all lie in the z-y plane? I know the magnitude of the moment is the cross product of force and the perpendicular distance from the force. So it should be F x r1, but what do you do about r2?

• i don't think this can be answered without giving full context of the picture, but it seems to me, that you can use it for computing the force by $\vec{F}=\vec{r_2}-\vec{r_1}$ if only $r_1$ and $r_2$ are given. Which is messy relationship, since force is measured in different units then radius vectors... – Umaxo May 15 at 5:29