1.A2.4 CENTRE OF GRAVITY (C of G)

The centre of gravity is the point at which the total weight of the body may be regarded as being concentrated. Another way of saying this is that the centre of gravity is the point about which the parts of a body exactly balance each other.

With a regularly shaped load the position of the centre of gravity can easily be judged by measuring out the midpoint in each direction. For more complex shapes, however, it may be necessary to estimate the centre of gravity of the various parts of the load and then combine them to get a centre of gravity for the whole.

If, in Figure 1.A2.4-1 , the weights of the three parts A,B and C are Wa, Wb and Wc respectively, then having estimated them and the positions of their centres of gravity, we can take moments about a given point (e.g. one end) and thus determine the position of the line through which the centre of gravity of the whole acts and which is an unknown distance X from the given point.

Thus from the formula (Wa + Wb + Wc) X = Wa Ya + Wb Yb + Wc Yc the unknown distance X can then easily be found.

Once the position has been estimated, it should be marked in some way (e.g. chalk, sticky tape) to guide the operative when attaching the slings.

This calculation will determine the position of the centre of gravity in one plane only. By applying the same method in the other two planes, the precise position can be determined.