Moment (Nm) = force (N) x perpendicular distance from pivot (m)
The perpendicular distance is basically the distance from the pivot to the line of force. It has to be 900 for the correct value.
An object will be in balance if: Sum of anticlockwise moments = sum of clockwise movements
Centre of Gravity
The centre of gravity is the point where the whole of the weight of the object appears of act.
It can be found by drawing lines of symmetry (if the object is rectangular or symmetrical) and finding the point which all the lines intersect, or, for an irregular shape, by suspending it freely by a point on its edge and using a plumb line to indicate the space directly vertical to the point in the I/GCSE physics.
The centre of gravity is always vertically below the pivot so you can work out the centre of gravity by drawing a point somewhere in the middle of the plumb line, or suspending the object at different points on its edge and finding out where all the lines intersect.
End of part I, remember to read part II !