Example $5(15$ points$)$

Problem $4\mathrm{.}90$ The force $F=10i+12j-6k(N).$

What is the moment of $F$ about the line $OA?$ Draw a

sketch to indicate the sense of the moment.

Example points$)$

$4\mathrm{.}115$ Determine the sum of the moments exerted on the plate by

the two couples.