ABSTRACT
Taking the moment of both sides of Eq.(2.15) with respect to the origin and summing over all particles, we have ( ) ∑ ∑∑∑
≠ ×+×=×
GGGGGG
In view of the previously used vector identity, the left-hand side gives the time rate of change of the total angular momentum, so that the equation reduces to
ijie FrGdt Ld
GGGG
where eG G
is the total external torque
eiie FrG GGG
and the double sum is the total internal torque, which vanishes if the line of action of all the internal forces ijF
G lie along the lines ijr
G joining the particles, as illustrated in Figure 3.1.
