To understand the notion of the centre of mass (CM) (or centre of gravity

(CG)) of a two-dimensional (plane) lamina, we start by considering a dis-

tribution of n point masses m1, m2, . . . , mn in the plane. Imagine the plane in which the masses are located to be horizontal, weightless and rigid. Then the

moment of these masses about a line L in the plane measures their combined turning effect about the line L . The magnitude ML of the moment is defined

to be

MLm1d1m2d2 . . .mndn,

where di is the perpendicular distance of mi from L , with di taken to be

positive when mi lies on one side of the line, and negative when it lies on the

other (the choice of which side is positive is arbitrary).