First 194 words of the document:
Moments are force x distance about a particular point
the units are Nm (Newton metres)
They either act in a clockwise or anti-clockwise direction.
Since moments are vectors one of these directions should be taken as positive.
If an object is said to be in equilibrium then two conditions must apply: the
sum of the moments about any given point must be zero. In other words, the
clockwise and anticlockwise moments will cancel each other out.
The sum of the forces acting on the object must be zero. In moments questions this
usually means the upwards forces cancel out the Downwards forces.
Sketch diagram showing all forces and distances
If the plank is resting on supports there will be a vertical reaction upwards at this
If the plank is uniform then the weight of the plank will act at its midpoint.
Take moments about a point , putting clockwise moments equal to anticlockwise
The line of action through the chosen point its force will be 0 so unknowns can be
removed from the equation.