The easiest way to take moments about a point
for a force not at right angles to the line joining the point of action
of the force to A is to resolve the force into components parallel to
and perpendicular to
The 50N force above has components
perpendicular toand
parallel to
The 80N force has components
perpendicular toand
parallel to
The moments of the forces are then
anticlockwise and
clockwise so the net moment due to the two forces is
The components of the forces parallel to
and
pass throughand do not contribute to the moment.