
Ch04-H6927.tex 20/6/2007 16: 33 Page 71
The Equations of Motion 71
Equations (4.12) represent the force equations of a generalised rigid body and
describe the motion of its cg since the origin of the axis system is co-located with the
cg in the body. In some applications, for example the airship, it is often convenient
to locate the origin of the axis system at some point other than the cg. In such cases
the condition described by equation (4.11) does not apply and equations (4.12) would
include rather more terms.
4.1.3 The generalised moment equations
Consider now the moments produced by the forces acting on the incremental mass
δm at point p(x, y,