MomentA moment is a force multiplied by a distance perpendicular from it’s line of action, i.e. the length of the “Moment Arm”.
When in static Equilibrium.
I.e. Anti-clockwise moments equals Clockwise Moments, so the since they should all cancel each-other out.
Line of Action
The Line of action is a invisible magical, often dotted line that extends out from a force of interest out to infinity.
If a force’s Line of action passes through your moment taking point, that force is not recorded.
This happens because the D in , Distance, is really just the length of a arm that extends out perpendicular from the line of action. It extends out from a point where it will intersect moment-taking point A.
The “moment arm” can be seen below.
Moments acting in the anti-clockwise direction should be negative.
E.g. The 10kN in the diagram above pushes in the Anti-clockwise Direction, where as 12kN and 20kN push into the clockwise direction.
Moment arms don’t always line up with the structure itself, they often go outside of it.
This happens mostly when forces are applied at an angle.
Moments at an angle
Syllabus“The Moment Arm, not the force, shall be the variable requiring trigonometry in determining any particular moment required.”
If a force is applied at an angle, to find the relevant moment we can use trig to make an imaginary triangle.
See Engineering Method of Sections Questions 19->22.pdf#page=1
The Syllabus also says that; ”All external forces are to be vertical only.”, so only Internal forces will be on an angle.
Though see question at the bottom of this page.
Moment Arms outside the Method of Sections
This can be re-written like this;
“Calculating the magnitude and direction of the resultant” is the same as usual;
“Calculating the Turning Moment” requires consideraiton of the line of action to determine forces that do not play apart.
And the fact that the length of the moment arm is apart of the Formula for a moment is used in a cool way in the last part.
