Problem 1.235

A force $\mathbf{F}=A \mathbf{i}+B \mathbf{j}$ is applied to a point whose radius vector relative to the origin of coordinates $O$ is equal to $\mathbf{r}=a \mathbf{i}+$ $+b \mathbf{j},$ where $a, b, A, B$ are constants, and $\mathbf{i}, \mathbf{j}$ are the unit vectors of the $x$ and $y$ axes. Find the moment $\mathbf{N}$ and the arm $l$ of the force $\mathbf{F}$ relative to the point $O$.