1. Moment of a force or Torque
Turning effects

2. The Moment of a force is a measure of its tendency to cause a body to rotate about a specific point or axis.
for a moment to develop, the force must act upon the body in such a manner that the body would begin to twist. This occurs every time a force is applied so that it does not pass through the centroid of the body.
A moment is due to a force not having an equal and opposite force directly along it's line of action.

3. Its unit is the Newton Metre, Nm
The magnitude of the moment of a force acting about a point or axis is directly proportional to the distance of the force from the point or axis. It is defined as the product of the force (F) and the perpendicular distance (d) between the line of action of the force and the center of moments.
Moment = Force x Distance or M = (F)(d)
Its unit is the Newton Metre, Nm
Taking moments about a the point of contact (the pivot) eliminates the need to find the moment due to the Normal force)

5. Principle of moments
States that for two bodies to be in equilibrium the sum of the clockwise moments is equal to the sum of the anticlockwise moments
Calculate dN

6. Couple
A Couple is a system of forces with a resultant (i.e. net, or sum) moment but no resultant force, also called torque but different to synonym for moment of a force.
Another term for a couple is a pure moment.
Its effect is to create rotation without translation, or more generally without any acceleration of the centre of mass.
Eg. Force exerted by hand on a screwdriver and force exerted by screwdriver tip on screw