The Definition of a Moment

Consider a force $F$ located a distance $a$ from the origin $O$ of a rectangular coordinate system (Fig. 1); $a$ is measured in the plane of the force $F$ and the point $O$, perpendicular to the line of action of $F$. The moment of $F$ about the point $O$, or about the axis through $O$ normal to the plane of $F$ and $O$, is defined as the vector $M$, whose magnitude is $(F)(a)$, whose direction is the line $OA$, normal to the plane of $F$ and $a$, and whose sense is the direction of advance of a right-handed screw rotated by the force $F$. This rule for sense may also be stated by saying that if one looks in the direction of $M$, $F$ will appear to turn clockwise around $M$.